drm/simple_kms_helper: Add {enable|disable}_vblank callback support

If simple_kms_helper based driver needs to work with vblanks,
then it has to provide drm_driver.{enable|disable}_vblank callbacks,
because drm_simple_kms_helper.drm_crtc_funcs does not provide any.
At the same time drm_driver.{enable|disable}_vblank callbacks
are marked as deprecated and shouldn't be used by new drivers.

Fix this by extending drm_simple_kms_helper.drm_crtc_funcs
to provide the missing callbacks.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1518425574-32671-2-git-send-email-andr2000@gmail.com