drm/i915: re-enable rc6 support for Ironlake+

Re-enable rc6 support on Ironlake for power savings.  Adds a debugfs
file to check current RC state, adds a missing workaround for Ironlake
MI_SET_CONTEXT instructions, and renames MCHBAR_RENDER_STANDBY to
RSTDBYCTL to match the docs.

Keep RC6 and the power context disabled on pre-ILK.  It only seems to
hang and doesn't save any power.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_suspend.c b/drivers/gpu/drm/i915/i915_suspend.c
index 4107724..af53063 100644
--- a/drivers/gpu/drm/i915/i915_suspend.c
+++ b/drivers/gpu/drm/i915/i915_suspend.c
@@ -740,7 +740,7 @@
 		I915_WRITE(PCH_PP_OFF_DELAYS, dev_priv->savePP_OFF_DELAYS);
 		I915_WRITE(PCH_PP_DIVISOR, dev_priv->savePP_DIVISOR);
 		I915_WRITE(PCH_PP_CONTROL, dev_priv->savePP_CONTROL);
-		I915_WRITE(MCHBAR_RENDER_STANDBY,
+		I915_WRITE(RSTDBYCTL,
 			   dev_priv->saveMCHBAR_RENDER_STANDBY);
 	} else {
 		I915_WRITE(PFIT_PGM_RATIOS, dev_priv->savePFIT_PGM_RATIOS);
@@ -811,7 +811,7 @@
 		dev_priv->saveFDI_RXA_IMR = I915_READ(FDI_RXA_IMR);
 		dev_priv->saveFDI_RXB_IMR = I915_READ(FDI_RXB_IMR);
 		dev_priv->saveMCHBAR_RENDER_STANDBY =
-			I915_READ(MCHBAR_RENDER_STANDBY);
+			I915_READ(RSTDBYCTL);
 	} else {
 		dev_priv->saveIER = I915_READ(IER);
 		dev_priv->saveIMR = I915_READ(IMR);