drm/i915/vlv: save/restore the power context base reg

Some BIOSes (e.g. the one on the Minnowboard) don't save/restore this
reg.  If it's unlocked, we can just restore the previous value, and if
it's locked (in case the BIOS re-programmed it for us) the write will be
ignored and we'll still have "did it move" sanity check in the PM code to
warn us if something is still amiss.

References: https://bugs.freedesktop.org/show_bug.cgi?id=89611
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Darren Hart <dvhart@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_drv.c

@@ -1095,6 +1095,7 @@ static void vlv_save_gunit_s0ix_state(struct drm_i915_private *dev_priv)
 	/* Gunit-Display CZ domain, 0x182028-0x1821CF */
 	s->gu_ctl0		= I915_READ(VLV_GU_CTL0);
 	s->gu_ctl1		= I915_READ(VLV_GU_CTL1);
+	s->pcbr			= I915_READ(VLV_PCBR);
 	s->clock_gate_dis2	= I915_READ(VLV_GUNIT_CLOCK_GATE2);
 
 	/*
@@ -1189,6 +1190,7 @@ static void vlv_restore_gunit_s0ix_state(struct drm_i915_private *dev_priv)
 	/* Gunit-Display CZ domain, 0x182028-0x1821CF */
 	I915_WRITE(VLV_GU_CTL0,			s->gu_ctl0);
 	I915_WRITE(VLV_GU_CTL1,			s->gu_ctl1);
+	I915_WRITE(VLV_PCBR,			s->pcbr);
 	I915_WRITE(VLV_GUNIT_CLOCK_GATE2,	s->clock_gate_dis2);
 }