drm/i915: update last_vblank when disabling the power well

The DRM layer keeps track of our vblanks and it assumes our vblank
counters only go back to zero when they overflow. The problem is that
when we disable the power well our counters also go to zero, but it
doesn't mean they did overflow. So on this patch we grab the lock and
update last_vblank so the DRM layer won't think our counters
overflowed.

This patch fixes the following intel-gpu-tools test:
./kms_flip --run-subtest blocking-absolute-wf_vblank

Regression introduced by the following commit:

commit bf51d5e2
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Wed Jul 3 17:12:13 2013 -0300
    drm/i915: switch disable_power_well default value to 1

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66808


Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Added a comment that this might be better done in
drm_vblank_post_modeset in general.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>