drm/msm: default to XR24 rather than AR24

Since X11 is going to create an XR24 fb, if the pixel formats do not
match then crtc helpers will think it is a full modeset even if mode is
the same, which prevents smooth/flickerless handover from fbcon/plymouth
to X11.

Signed-off-by: Rob Clark <robdclark@gmail.com>