drm/i915: Get power refs in encoder->get_power_domains()

Push getting the reference for the encoders' power domains into the
encoder get_power_domains() hook instead of doing this from the caller.
This way the encoder can store away the corresponding wakerefs.

This fixes the DSI encoder disabling, which didn't release these
power references it acquired during HW state readout.

Note that longtime ownership for the corresponding wakerefs can be thus
acquired / released in two ways. Nevertheless there is always only one
owner for them:

After HW readout (booting/system resume):
- encoder->get_power_domains() acquires
- encoder->disable*() releases

After a modeset (calling intel_atomic_commit()):
- encoder->enable*() acquires
- encoder->disable*() releases

* can be any of the encoder enable/disable hooks.

v2:
- Check that the DSI io_wakerefs are unset both during encoder HW
  readout and enabling. (Chris)

Fixes: 0e6e0be4 ("drm/i915: Markup paired operations on display power domains")
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
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/20190407124655.31536-1-imre.deak@intel.com