drm/i915: set ctx->initialized only after RCS

The initialized flag is used to specify a context has been initialized
and it's context is safe to load, ie. the 3d state is setup properly.
With full PPGTT, we emit the address space loads during context switch
and this currently marks a context as initialized. With full PPGTT
patches, if a client first emits a batch to !RCS, then later, RCS, the
code will mistake the context as initialized and try to reload an
uninitialized context.

1. context 1 blit // context marked as initialized, but isn't
2. context 1 render // loads random state from step 2

It is really easy to hit this with a planned upcoming patch which makes
default context reuse possible.

NOTE: This should only effect full PPGTT branches, ie. current
drm-intel-nightly.

Thanks to Chris for helping me track this down.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
[danvet: Simplify the failure scenario in the commit message according
to Chris' review a bit.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -692,10 +692,11 @@ static int do_switch(struct intel_ring_buffer *ring,
 		i915_gem_context_unreference(from);
 	}
 
+	to->is_initialized = true;
+
 done:
 	i915_gem_context_reference(to);
 	ring->last_context = to;
-	to->is_initialized = true;
 	to->last_ring = ring;
 
 	return 0;