drm: shmobile: use vblank hooks in struct drm_crtc_funcs

The vblank hooks in struct drm_driver are deprecated and only meant for
legacy drivers.  For modern drivers with DRIVER_MODESET flag, the hooks
in struct drm_crtc_funcs should be used instead.

As the result, shmob_drm_crtc_enable_vblank() becomes a static function,
although it gets moved around a bit to save forward declaration.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486458995-31018-19-git-send-email-shawnguo@kernel.org