drm/i915: Set AGPBUSY# bit in init_clock_gating

I don't see why we wouldn't want interrupts to wake up the CPU from C3
always, so just set the AGPBUSY# bit in gen3_init_clock_gating().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>