drm/amd/display: Remove unused headers

They're not being used, so remove them.

Signed-off-by: Imad Syed <imad.syed@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>