drm/i915: Rename intel_context.active to .inflight

Rename the engine this HW context is currently active upon (that we are
flying upon) to disambiguate between the mixture of different active
terms (and prevent conflict in future patches).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528092956.14910-14-chris@chris-wilson.co.uk