drm/i915: Eliminate per-fw_domain i915 backpointer

Pass along the drm_i915_private pointer from the caller, rather than
looking it up from each fw_domain during fw_domains_get/_put. This
allows us to then eliminate the backpointer, in exchange for a more
complicated unwrapping procedure in the rare
intel_uncore_fw_release_timer().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323101944.21627-1-chris@chris-wilson.co.uk