drm/i915: Revert workaround for disabling L3 cache aging on IVB

In commit e4e0c058
Author: Eugeni Dodonov <eugeni.dodonov@intel.com>
Date:   Wed Feb 8 12:53:50 2012 -0800

    drm/i915: gen7: Implement an L3 caching workaround.

the L3 cache aging was disabled. This was part of a shotgun response
to a number of GPU hang bugs, but there appears to be no documentation
to suggest that disabling the L3 cache age was ever required (to prevent
the GPU hangs).

Restoring the L3 cache age is a minor performance win of around 2%
on IVB:GT2. (Note that this value seems to be consistent across a number
of tests and so appears to be above the usual noise.)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_reg.h

@@ -4169,7 +4169,7 @@
 #define  VLV_B0_WA_L3SQCREG1_VALUE		0x00D30000
 
 #define GEN7_L3CNTLREG1				0xB01C
-#define  GEN7_WA_FOR_GEN7_L3_CONTROL			0x3C4FFF8C
+#define  GEN7_WA_FOR_GEN7_L3_CONTROL			0x3C47FF8C
 #define  GEN7_L3AGDIS				(1<<19)
 
 #define GEN7_L3_CHICKEN_MODE_REGISTER		0xB030