drm/i915: support low power watermarks on Ironlake

This patch actually makes the watermark code even uglier (if that's
possible), but has the advantage of sharing code between SNB and ILK at
least.  Longer term we should refactor the watermark stuff into its own
file and clean it up now that we know how it's supposed to work.

Supporting WM2 on my Vaio reduced power consumption by around 0.5W, so
this patch is definitely worthwhile (though it also needs lots of test
coverage).
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: pass the watermark structs arounds]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_reg.h

@@ -2345,8 +2345,13 @@
 
 /* Memory latency timer register */
 #define MLTR_ILK		0x11222
+#define  MLTR_WM1_SHIFT		0
+#define  MLTR_WM2_SHIFT		8
 /* the unit of memory self-refresh latency time is 0.5us */
 #define  ILK_SRLT_MASK		0x3f
+#define ILK_LATENCY(shift)	(I915_READ(MLTR_ILK) >> (shift) & ILK_SRLT_MASK)
+#define ILK_READ_WM1_LATENCY()	ILK_LATENCY(MLTR_WM1_SHIFT)
+#define ILK_READ_WM2_LATENCY()	ILK_LATENCY(MLTR_WM2_SHIFT)
 
 /* define the fifo size on Ironlake */
 #define ILK_DISPLAY_FIFO	128