Originally these functions have been for user modesetting drivers to
ensure vblank processing doesn't fall over completely around modeset
changes. This has been carried over ever since then.

Now that Ville cleaned our vblank handling with an explicit
drm_vblank_off/on braket when disabling/enabling crtcs. So this seems
to be unnecessary now. The most important side effect was that due to
the delayed vblank disabling we have been pretty much guaranteed to
receive a vblank interrupt soonish after a crtc was enabled.

Note that our vblank handling across modeset is still fairly decent
fubar - we don't actually handle vblank counter all to well.
drm_update_vblank_count will make sure that the frame counter always
rolls forward, but userspace isn't really all to ready to cope with
the big jumps this causes.

This isn't a big mostly because the hardware retains the frame
counter. But with runtime pm and also across suspend/resume we fall
over.

Fixing this is a lot more involved and also needs som i-g-ts. So
material for another patch series.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>