drm/amd/pm: Add empty string validation to sysfs store functions

drm/amd/pm: Add empty string validation to sysfs store functions

Discovery: Fuzzing for secure supply chain requirements
Tool: amd_fuzzing_sysfs (IGT test)

The AMDGPU power management sysfs store functions accept whitespace-only
strings when they should reject them with -EINVAL. This was discovered via
systematic fuzzing of sysfs interfaces crossing the user/kernel trust
boundary.

Affected functions:
- amdgpu_set_power_dpm_force_performance_level (power_dpm_force_performance_level)
- amdgpu_set_power_dpm_state (power_dpm_state)
- amdgpu_set_pp_power_profile_mode (pp_power_profile_mode)
- amdgpu_read_mask (used by pp_dpm_sclk/mclk/fclk/socclk/pcie)
- amdgpu_set_pp_features (pp_features)

Impact:
- Whitespace-only writes (e.g., "\n", " ") can cause unexpected behavior
- Better input validation at user/kernel trust boundary
- Defense-in-depth improvement

Root Cause:
The sysfs_streq() function matches whitespace-only strings against empty
string, allowing invalid input to be processed.

Fix:
Add explicit validation at the start of each affected store function:

    if (count == 0 || sysfs_streq(buf, ""))
        return -EINVAL;

This rejects whitespace-only inputs before they are processed. Note that
write() calls with count=0 (truly empty strings) are handled by the VFS
layer before reaching the sysfs .store() callback - the VFS returns 0
(success) without calling the kernel function. This is POSIX-compliant
behavior and cannot be changed at the kernel driver level.

What This Patch Fixes:
- Whitespace-only strings: "\n", " ", "  ", etc. are now rejected
- Defense-in-depth: Explicit validation at trust boundary
- Code clarity: Intent to reject invalid input is explicit

What This Patch Cannot Fix:
- write(fd, "", 0) returning success - this is VFS layer behavior
- Fuzzer tests for empty strings (count=0) will still report "accepted"
  because the VFS handles this before the kernel callback

Test Results After Fix:
- Whitespace strings ("\n", " ") now properly rejected
- Empty string tests (count=0) still show as "accepted" due to VFS behavior
- Overall improvement in input validation robustness
- No impact on valid inputs

This is a defense-in-depth improvement that hardens input validation
even though VFS layer behavior prevents catching all edge cases.

Tested: amd_fuzzing_sysfs IGT test

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Jesse Zhang <jesse.zhang@amd.com>
Signed-off-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>