drm/i915: don't frob the vblank ts in finish_page_flip

Now that we correctly generate it, this hack is no longer required (and
might actually paper over a serious bug).

pageflip timestamps are sanity check in the latest version of the flip-test
in intel-gpu-tools.

v2: Also remove the gettimeofday(&now) which is no longer used.
Noticed by Mario Kleiner.

Reviewed-by:  <mario.kleiner@tuebingen.mpg.de>
Tested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>