drm/mxsfb: Drop drm_vblank_cleanup

Almost right but still racy, it's called before the interrupts are
uninstalled. So let's just drop it.

Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621082850.13224-7-daniel.vetter@ffwll.ch