drm/i915: Name the inner most per-engine intel_context struct

We want to give a name to the currently anonymous per-engine struct
inside the context, so that we can assign it to a local variable and
save clumsy typing. The name we have chosen is intel_context as it
reflects the HW facing portion of the context state (the logical context
state, the registers, the ringbuffer etc).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464098023-3294-4-git-send-email-chris@chris-wilson.co.uk