drm/amdgpu/display: drop set but unused variable

No longer used so drop it.

Fixes: ec457f83 ("drm/amd/display: Drop unnecessary detect link code")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>