drm: tilcdc: use vblank hooks in struct drm_crtc_funcs

drm: tilcdc: 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.

While at it, the 'return' of .disable_vblank is dropped to fix the
following checkpatch warning.

 WARNING: void function return statements are not generally useful

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Jyri Sarha <jsarha@ti.com>
Acked-by: Jyri Sarha <jsarha@ti.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486458995-31018-22-git-send-email-shawnguo@kernel.org