drm/i915: add get_vblank_counter function for GM45

As discussed in the long thread about vblank related timeouts, it turns out
GM45 has different frame count registers than previous chips.  This patch
adds support for them, which prevents us from waiting on really stale
sequence values in drm_wait_vblank (which rather than returning immediately
ends up timing out or getting interrupted).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/gpu/drm/i915/i915_reg.h

@@ -1371,6 +1371,9 @@
 #define   PIPE_FRAME_LOW_SHIFT    24
 #define   PIPE_PIXEL_MASK         0x00ffffff
 #define   PIPE_PIXEL_SHIFT        0
+/* GM45+ just has to be different */
+#define PIPEA_FRMCOUNT_GM45	0x70040
+#define PIPEA_FLIPCOUNT_GM45	0x70044
 
 /* Cursor A & B regs */
 #define CURACNTR		0x70080
@@ -1439,6 +1442,9 @@
 #define PIPEBSTAT		0x71024
 #define PIPEBFRAMEHIGH		0x71040
 #define PIPEBFRAMEPIXEL		0x71044
+#define PIPEB_FRMCOUNT_GM45	0x71040
+#define PIPEB_FLIPCOUNT_GM45	0x71044
+
 
 /* Display B control */
 #define DSPBCNTR		0x71180