Factor out helper functions to get/put a set of power domains that are
tracked using their wakeref handles. The same is needed by the next
patch adding tracking for enabled CRTC power domains.

v2: s/uint64_t/u64/ (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201201161340.2879202-1-imre.deak@intel.com