drm/amd/display: Unhardcode cursor size reported back to UMD.

This will return back MAX cursor size for given ASIC.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Jordan Lazare <Jordan.Lazare@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>