drm/amdgpu/display: drop unused variable

Remove unused variable.

Fixes: e7d9560a

 ("Revert "drm/amd/display: Fix overlay validation by considering cursors"")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>