imx-drm: imx-drm-core: use the crtc drm device for vblank

imx-drm: imx-drm-core: use the crtc drm device for vblank

There are a couple of ways to get at the drm_device for the vblank
operations.  One of them is via the private imxdrm structure, the
other is via the DRM crtc structure, which also stores a pointer.
Use the DRM method instead of our own method.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>