drm/i915: Don't ERROR for an expected intel_rcs_ctx_init() interruption

intel_rcs_ctx_init() can be interrupted by a signal (if it has to wait
upon a full ring to advance). Don't emit an error for this.

Testcase: igt/gem_concurrent_blit
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1454086145-16160-3-git-send-email-chris@chris-wilson.co.uk