drm/amd/display: Enable gcov coverage for amdgpu_dm KUnit builds

drm/amd/display: Enable gcov coverage for amdgpu_dm KUnit builds

[Why & How]
Add GCOV_PROFILE := y to amdgpu_dm/Makefile under the
CONFIG_DRM_AMD_DC_KUNIT_TEST guard so coverage instrumentation
is only applied during KUnit builds, not all amdgpu_dm builds.

Add coverage config options (CONFIG_GCOV, CONFIG_DEBUG_INFO,
CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT, CONFIG_DEBUG_KERNEL)
to tests/.kunitconfig so the kunit.py runner enables the kernel
coverage configuration automatically.

Assisted-by: Copilot:Claude-Sonnet-4.6
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>