drm/amd/display: Run FP-disabled MST stub test only without DC FP

drm/amd/display: Run FP-disabled MST stub test only without DC FP

dm_mst_test_fp_guarded_public_stubs() passes NULL arguments to
dm_dp_mst_is_port_support_mode() to test the stub implementation when
CONFIG_DRM_AMD_DC_FP is disabled.

However, this test is always registered. When CONFIG_DRM_AMD_DC_FP is
enabled, the real implementation is used instead of the stub. The real
implementation expects valid pointers, so passing NULL can lead to a
NULL pointer dereference.

Register this test only when CONFIG_DRM_AMD_DC_FP is disabled.

Reported-by: Dan Carpenter <error27@gmail.com>
Cc: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Cc: Alex Hung <alex.hung@amd.com>
Cc: George Zhang <george.zhang@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>