drm/vkms: Convert to CRTC VBLANK callbacks

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

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Rodrigo Siqueira <rodrigosiqueira@gmail.com>
Tested-by: Rodrigo Siqueira <rodrigosiqueira@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123135943.24140-20-tzimmermann@suse.de