drm/i915: Initialize legacy semaphores from engine hw id indexed array

Build the legacy semaphore initialisation array using the engine
hardware ids instead of driver internal ones. This makes the
static array size dependent only on the number of gen6 semaphore
engines.

Also makes the per-engine semaphore wait and signal tables
hardware id indexed saving some more space.

v2: Refactor I915_GEN6_NUM_ENGINES to GEN6_SEMAPHORE_LAST. (Chris Wilson)
v3: More polish. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1471363461-9973-1-git-send-email-tvrtko.ursulin@linux.intel.com
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 9d723c2..86612d5 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -278,11 +278,14 @@
 		u32	sync_seqno[I915_NUM_ENGINES-1];
 
 		union {
+#define GEN6_SEMAPHORE_LAST	VECS_HW
+#define GEN6_NUM_SEMAPHORES	(GEN6_SEMAPHORE_LAST + 1)
+#define GEN6_SEMAPHORES_MASK	GENMASK(GEN6_SEMAPHORE_LAST, 0)
 			struct {
 				/* our mbox written by others */
-				u32		wait[I915_NUM_ENGINES];
+				u32		wait[GEN6_NUM_SEMAPHORES];
 				/* mboxes this ring signals to */
-				i915_reg_t	signal[I915_NUM_ENGINES];
+				i915_reg_t	signal[GEN6_NUM_SEMAPHORES];
 			} mbox;
 			u64		signal_ggtt[I915_NUM_ENGINES];
 		};