drm/i915: call drm_handle_vblank before finish_page_flip

... since finish_page_flip needs the vblank timestamp generated
in drm_handle_vblank. Somehow all the gmch platforms get it right,
but all the pch platform irq handlers get is wrong. Hooray for copy&
pasting!

Currently this gets papered over by a gross hack in finish_page_flip.
A second patch will remove that.

Note that without this, the new timestamp sanity checks in flip_test
occasionally get tripped up, hence the cc: stable tag.

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