drm/i915: Track power reference taken to disable power well functionality

Add wakeref tracking for the display power domain reference taken to
keep the display power well functionality disabled.

v2: Add missing wakeref zeroing to intel_power_domains_driver_remove()

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201201161340.2879202-2-imre.deak@intel.com