drm/i915: Optimize gen8_enable|disable_vblank functions

Let's cache the IMR value like on other platforms. This is needed to
implement the underrun reporting since then we'll have two places that
change the same register at runtime.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>