drm/i915/ddi: Track power reference taken for encoder DDI IO use

Add wakeref tracking for the DDI encoders' DDI_IO display power domain
references.

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/20201130212200.2811939-5-imre.deak@intel.com