drm/i915/gtt: Introduce release_pd_entry

By encapsulating the locking upper level and used check for entry
into a helper function, we can use it in all callsites.

v2: get rid of atomic_reads on lower level clears (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190705215204.4559-4-chris@chris-wilson.co.uk