drm/amd/display: Early return on crc get

If crc is disabled, early return when getting crc's. That way, we avoid
reading extra registers within the pflip high irq.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>