drm/i915: Set the context before setting up regs for the context.

Fixes failures in transform feedback on gen7 because our SOL_RESET
flag was setting the transform feedback offsets in the old context
(occasionally happened to be ours) instead of the new context.
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1228,6 +1228,10 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
 		}
 	}
 
+	ret = i915_switch_context(ring, file, ctx_id);
+	if (ret)
+		goto err;
+
 	if (ring == &dev_priv->ring[RCS] &&
 	    mode != dev_priv->relative_constants_mode) {
 		ret = intel_ring_begin(ring, 4);
@@ -1249,10 +1253,6 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
 			goto err;
 	}
 
-	ret = i915_switch_context(ring, file, ctx_id);
-	if (ret)
-		goto err;
-
 	trace_i915_gem_ring_dispatch(ring, seqno);
 
 	exec_start = batch_obj->gtt_offset + args->batch_start_offset;