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>
diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
index b86edac..71de9ee 100644
--- a/drivers/gpu/drm/i915/intel_pm.c
+++ b/drivers/gpu/drm/i915/intel_pm.c
@@ -5504,6 +5504,9 @@
 
 	/* IIR "flip pending" means done if this bit is set */
 	I915_WRITE(ECOSKPD, _MASKED_BIT_DISABLE(ECO_FLIP_DONE));
+
+	/* interrupts should cause a wake up from C3 */
+	I915_WRITE(INSTPM, _MASKED_BIT_DISABLE(INSTPM_AGPBUSY_DIS));
 }
 
 static void i85x_init_clock_gating(struct drm_device *dev)