drm/i915: Split out parking from the idle worker for reuse

drm/i915: Split out parking from the idle worker for reuse

We will want to park GEM before disengaging the drive^W^W^W unwedging.
Since we already do the work for idling, expose the guts as a new
function that we can then reuse.

v2: Just skip if already parked; makes it more forgiving to use by
future callers.
v3: Extract mark_busy, rename it to i915_gem_unpark and place it next to
i915_gem_park so that we can evaluate it for symmetry more easily.
Calling GEM from inside i915_request looks to be a bit of a layering
violation, for the moment I am imaging them as being notify_cb.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Michal Wajdeczko <[email protected]>
Cc: Sagar Arun Kamble <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]> #v1
Reviewed-by: Michal Wajdeczko <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]