drm: Make the decision to keep vblank irq enabled earlier

We want to provide the vblank irq shadow for pageflip events as well as
vblank queries. Such events are completed within the vblank interrupt
handler, and so the current check for disabling the irq will disable it
from with the same interrupt as the last pageflip event. If we move the
decision on whether to disable the irq (based on there no being no
remaining vblank events, i.e. vblank->refcount == 0) to before we signal
the events, we will only disable the irq on the interrupt after the last
event was signaled. In the normal course of events, this will keep the
vblank irq enabled for the entire flip sequence whereas before it would
flip-flop around every interrupt.

v2: Move the disable_fn() call outside of the vblank_event_lock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Dave Airlie <airlied@redhat.com>,
Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> #v1
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170324173058.23051-1-chris@chris-wilson.co.uk