The drm core has a nice ready-made helper for exactly the simple case
where it should fire on the next vblank.

Note that arming the vblank event in _begin is probably too early, and
might easily result in the vblank firing too early, before the new set
of planes are actually disabled. But that's kinda a minor issue
compared to just outright hanging userspace.

v2: Be more robust and either arm, when the CRTC is on, or just send
the event out right away.

v3: Just unconditionally send out the event directly, for safety -
arcpgu doesn't even have vblank support ...

Cc: Carlos Palminha <palminha@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: linux-snps-arc@lists.infradead.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-3-git-send-email-daniel.vetter@ffwll.ch
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-4-git-send-email-daniel.vetter@ffwll.ch