drm/imx: Switch to drm_fb_cma_prepare_fb() helper

Remove the common code from the driver and use the
drm_fb_cma_prepare_fb() helper instead.
Moveover, call the helper from prepare_fb() plane hook .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114100732.3446-2-marex@denx.de
Link: http://patchwork.freedesktop.org/patch/msgid/1476451342-146510-1-git-send-email-dvyukov@google.com