drm/imx: Remove imx_drm_crtc_vblank_get/_put()

There is no one calling imx_drm_crtc_vblank_get/_put() and
they are just two simple wrappers of drm_crtc_vblank_get/_put()
without doing any thing fancy - the drivers may call
drm_crtc_vblank_get/_put() directly.  So, let's remove the two
wrappers.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>