drm/i915: implement WaDisablePSDDualDispatchEnable on IVB & VLV

Workaround for dual port PS dispatch on GT1.

v2: pull in register definition & offset handling
v3: use IVB GT1 macro to get the right regs (Ben)
v4: add for VLV too (Ben)
v5: don't read the reg, it's masked so we'll only enable the one extra bit (Chris)
v6: use a _GT2 suffix for the second reg (Chris)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Antti Koskipää <antti.koskipaa@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index ba74df2..5138563 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -1134,6 +1134,7 @@
 
 	switch (reg) {
 	case GEN7_ROW_CHICKEN2:
+	case GEN7_HALF_SLICE_CHICKEN1:
 		return false;
 	default:
 		break;