drm/i915: Use I915_READ_FW in i915_get_vblank_counter()

Optimize the multi-register read in i915_get_vblank_counter() a little
bit by grabbing the uncore lock manually and using I915_READ_FW().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170309154434.29303-2-ville.syrjala@linux.intel.com


Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>