drm/msm/disp: simplify RGB{,A,X} formats definitions

drm/msm/disp: simplify RGB{,A,X} formats definitions

Define several additional macros, capturing RGB format classes, in order
to simplify defining particular RGB* format.

Reviewed-by: Jessica Zhang <jessica.zhang@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/688168/
Link: https://lore.kernel.org/r/20251114-dpu-formats-v3-4-cae312379d49@oss.qualcomm.com
Tested-by: Luca Weiss <luca.weiss@fairphone.com> # qcm6490-fairphone-fp5
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>