drm/i915: factor out reset_vblank_counter

We need to do the same for other platforms in upcoming patches.

v2:
- s/p/pipe (Ville)
- Call the new helper with the vbl_lock already held. The part it
  protects is short, so releasing it between pipes only makes proving
  correctness more difficult.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Resolve conflict with Damien's s/p/pipe/ change.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>