drm/mxsfb: Use simple_display_pipe prepare_fb helper

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180405154449.23038-6-daniel.vetter@ffwll.ch