drm/gma500: Add missing call to allow enabling vblank on psb/cdv

This adds a missing call to drm_crtc_vblank_on to the common DPMS helper
(used by poulsbo and cedartrail), which is called in the CRTC enable path.

With that call, it becomes possible to enable vblank when needed.
It is already balanced by a drm_crtc_vblank_off call in the helper.

Other platforms (oaktrail and medfield) use a dedicated DPMS helper that
does not have the proper vblank on/off hooks. They are not added in this
commit due to lack of hardware to test it with.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106094400.445834-2-paul.kocialkowski@bootlin.com