drm/vblank: Fix flip event vblank count

On machines where the vblank interrupt fires some time after the start
of vblank (or we just manage to race with the vblank interrupt handler)
we will currently stuff a stale vblank counter value into the flip event,
and thus we'll prematurely complete the flip.

Switch over to drm_crtc_accurate_vblank_count() to make sure we have an
up to date counter value, crucially also remember to add the +1 so that
the delayed vblank interrupt won't complete the flip prematurely.

Cc: stable@vger.kernel.org
Cc: Daniel Vetter <daniel@ffwll.ch>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010133322.24029-1-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel@ffwll.ch> #irc