drm/sti: Convert to CRTC VBLANK callbacks

VBLANK callbacks in struct drm_driver are deprecated in favor of
their equivalents in struct drm_crtc_funcs. Convert sti over.

v2:
	* remove unnecessary include of sti_crtc.h from sti_drv.c

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123135943.24140-17-tzimmermann@suse.de