drm/amd/display: rename BGRA8888 to ABGR8888

DC actually support ABGR8888 instead of BGRA8888 (R/B swap rather than endian swap) ,
rename to avoid confusion

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>