drm: mxsfb: Enable vblank handling

Enable vblank handling when the CRTC is turned on and disable it when it
is turned off. This requires moving vblank init after the KMS pipeline
initialisation, otherwise drm_vblank_init() gets called with 0 CRTCs.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-15-laurent.pinchart@ideasonboard.com