drm/mxsfb: Do not use deprecated drm_driver.{enable|disable)_vblank

Do not use deprecated drm_driver.{enable|disable)_vblank callbacks,
but use drm_simple_kms_helpe's pipe callbacks instead.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1518425574-32671-3-git-send-email-andr2000@gmail.com