drm/amd/display: Clean up SPL code

[Why & How]
Use helper functions for checking formats
Apply cositing offset in rotation case

Reviewed-by: Navid Assadian <navid.assadian@amd.com>
Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>