drm/i915: Generalise BSD default selection

For the default I915_EXEC_BSD round robin selector, it may select any
available VCS engine. Make it so.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190809091010.23281-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -2120,7 +2120,8 @@ gen8_dispatch_bsd_engine(struct drm_i915_private *dev_priv,
 
 	/* Check whether the file_priv has already selected one ring. */
 	if ((int)file_priv->bsd_engine < 0)
-		file_priv->bsd_engine = get_random_int() & 1;
+		file_priv->bsd_engine =
+			get_random_int() % num_vcs_engines(dev_priv);
 
 	return file_priv->bsd_engine;
 }