drm/i915: Fix the GT fence revocation runtime PM logic

To optimize some task deferring it until runtime resume unless someone
holds a runtime PM reference (because in this case the task can be done
w/o the overhead of runtime resume), we have to use the runtime PM
get-if-active logic: If the runtime PM usage count is 0 (and so
get-if-in-use would return false) the runtime suspend handler is not
necessarily called yet (it could be just pending), so the device is not
necessarily powered down, and so the runtime resume handler is not
guaranteed to be called.

The fence revocation depends on the above deferral, so add a
get-if-active helper and use it during fence revocation.

v2:
- Add code comment explaining the fence reg programming deferral logic
  to i915_vma_revoke_fence(). (Chris)
- Add Cc: stable and Fixes: tags. (Chris)
- Fix the function docbook comment.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <stable@vger.kernel.org> # v4.12+
Fixes: 181df2d4 ("drm/i915: Take rpm wakelock for releasing the fence on unbind")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210322204223.919936-1-imre.deak@intel.com