drm/imx: Remove imx_drm_handle_vblank()

imx_drm_handle_vblank() is just a simple wrapper of drm_crtc_handle_vblank()
without doing any thing fancy - drm_crtc_handle_vblank() can be called
directly.  So, let's remove the wrapper.

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