drm/amd/display: validate plane in dce110 for scaling

CZ & ST support uptil a limit 2:1 downscaling, this patch
adds validate_plane hook, that shall be used to validate
the plane attributes sent by the user space based
on dce110 capabilities.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>