drm/vc4: Enable/Disable vblanks properly in crtc en/disable.

Add missing drm_crtc_vblank_on/off() calls so vblank irq
handling/updating/timestamping never runs with a crtc shut down
or during its shutdown/startup, as that causes large jumps in
vblank count and trouble for compositors.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>