As of commit 6c80a93be62d398e ("drm/fb-helper: Initialize fb-helper's
preferred BPP in prepare function"), the preferred_bpp parameter of
drm_fb_helper_prepare() defaults to 32 instead of
drm_mode_config.preferred_depth. Hence this also applies to
drm_fbdev_dma_setup(), which just passes its own preferred_bpp
parameter.