drm/i915: GEN7_MSG_CONTROL is ivb-only

At least I couldn't find it in the Haswell Bspec any more and we've
tried to test-boot a Haswell machine with num_pipes forced to 0 (i.e.
hit the PCH_NOP path) and the unclaimed register logic complained.

So restrict this dance to just ivb platforms.

v2: Art pointed out that the bits simply moved on hsw+

v3: Buy code terseneness with a notch of sublety as suggested by
Chris.

v4: Frob the right bit, spotted by Art.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Arthur Ranyan <arthur.j.runyan@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Reviewed-by: Art Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -4471,9 +4471,15 @@ i915_gem_init_hw(struct drm_device *dev)
 			   LOWER_SLICE_ENABLED : LOWER_SLICE_DISABLED);
 
 	if (HAS_PCH_NOP(dev)) {
+		if (IS_IVYBRIDGE(dev)) {
 			u32 temp = I915_READ(GEN7_MSG_CTL);
 			temp &= ~(WAIT_FOR_PCH_FLR_ACK | WAIT_FOR_PCH_RESET_ACK);
 			I915_WRITE(GEN7_MSG_CTL, temp);
+		} else if (INTEL_INFO(dev)->gen >= 7) {
+			u32 temp = I915_READ(HSW_NDE_RSTWRN_OPT);
+			temp &= ~RESET_PCH_HANDSHAKE_ENABLE;
+			I915_WRITE(HSW_NDE_RSTWRN_OPT, temp);
+		}
 	}
 
 	i915_gem_init_swizzling(dev);

drivers/gpu/drm/i915/i915_reg.h

@@ -4119,6 +4119,8 @@
 #define GEN7_MSG_CTL	0x45010
 #define  WAIT_FOR_PCH_RESET_ACK		(1<<1)
 #define  WAIT_FOR_PCH_FLR_ACK		(1<<0)
+#define HSW_NDE_RSTWRN_OPT	0x46408
+#define  RESET_PCH_HANDSHAKE_ENABLE	(1<<4)
 
 /* GEN7 chicken */
 #define GEN7_COMMON_SLICE_CHICKEN1		0x7010