drm/i915: Decouple execbuf uAPI from internal implementation

At the moment execbuf ring selection is fully coupled to
internal ring ids which is not a good thing on its own.

This dependency is also spread between two source files and
not spelled out at either side which makes it hidden and
fragile.

This patch decouples this dependency by introducing an explicit
translation table of execbuf uAPI to ring id close to the only
call site (i915_gem_do_execbuffer).

This way we are free to change driver internal implementation
details without breaking userspace. All state relating to the
uAPI is now contained in, or next to, i915_gem_do_execbuffer.

As a side benefit, this patch decreases the compiled size
of i915_gem_do_execbuffer.

v2: Extract ring selection into eb_select_ring. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452870770-13981-1-git-send-email-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/intel_ringbuffer.h

@@ -149,14 +149,14 @@ struct  i915_ctx_workarounds {
 struct  intel_engine_cs {
 	const char	*name;
 	enum intel_ring_id {
-		RCS = 0x0,
-		VCS,
+		RCS = 0,
 		BCS,
-		VECS,
-		VCS2
+		VCS,
+		VCS2,	/* Keep instances of the same type engine together. */
+		VECS
 	} id;
 #define I915_NUM_RINGS 5
-#define LAST_USER_RING (VECS + 1)
+#define _VCS(n) (VCS + (n))
 	u32		mmio_base;
 	struct		drm_device *dev;
 	struct intel_ringbuffer *buffer;