drm/i915/gt: Update request status flags for debug pretty-printer

We plan to expand upon the number of available statuses for when we
pretty-print the requests along the timelines, and so need a new set of
flags. We have settled upon:

	Unready [U]
	  - initial status after being submitted, the request is not
	    ready for execution as it is waiting for external fences

	Ready [R]
	  - all fences the request was waiting on have been signaled,
            and the request is now ready for execution and will be
	    in a backend queue

	  - a ready request may still need to wait on semaphores
	    [internal fences]

	Ready/virtual [V]
	  - same as ready, but queued over multiple backends

	Executing [E]
	  - the request has been transferred from the backend queue and
	    submitted for execution on HW

	  - a completed request may still be regarded as executing, its
	    status may not be updated until it is retired and removed
	    from the lists

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201119165616.10834-3-chris@chris-wilson.co.uk