drm/i915: Use drm official vblank_no_hw_counter callback.

No functional change. Instead of defining a new empty function
let's use what is available on drm.

It gets cleaner, and easy to read, and understand.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>