drm/i915: Rename power_domains.wakeref to init_wakeref

Rename power_domains.wakeref to power_domains.init_wakeref to make the
use of this reference clearer. The next patch adds tracking for another
power reference user of the power_domains functionality.

While at it add a missing zero wakeref assert when setting the wakeref.

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/20201130212200.2811939-8-imre.deak@intel.com