drm/i915: Allow the user to pass a context to any ring

With full-ppgtt, we want the user to have full control over their memory
layout, with a separate instance per context. Forcing them to use a
shared memory layout for !RCS not only duplicates the amount of work we
have to do, but also defeats the memory segregation on offer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20160822080350.4964-4-chris@chris-wilson.co.uk
Reviewed-by: John Harrison <john.c.harrison@intel.com>
Reviewed-by: Thomas Daniel <thomas.daniel@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1253,12 +1253,9 @@ static struct i915_gem_context *
 i915_gem_validate_context(struct drm_device *dev, struct drm_file *file,
 			  struct intel_engine_cs *engine, const u32 ctx_id)
 {
-	struct i915_gem_context *ctx = NULL;
+	struct i915_gem_context *ctx;
 	struct i915_ctx_hang_stats *hs;
 
-	if (engine->id != RCS && ctx_id != DEFAULT_CONTEXT_HANDLE)
-		return ERR_PTR(-EINVAL);
-
 	ctx = i915_gem_context_lookup(file->driver_priv, ctx_id);
 	if (IS_ERR(ctx))
 		return ctx;