drm/i915: Don't explode when the dig port we don't have an AUX CH

The icl+ power well code currently assumes that every AUX power
well maps to an encoder which is using said power well. That is
by no menas guaranteed as we:
- only register encoders for ports declared in the VBT
- combo PHY HDMI-only encoder no longer get an AUX CH since
  commit 9856308c ("drm/i915: Only populate aux_ch if really needed")

However we have places such as intel_power_domains_sanitize_state()
that blindly traverse all the possible power wells. So these bits
of code may very well encounbter an aux power well with no associated
encoder.

In this particular case the BIOS seems to have left one AUX power
well enabled even though we're dealing with a HDMI only encoder
on a combo PHY. We then proceed to turn off said power well and
explode when we can't find a matching encoder. As a short term fix
we should be able to just skip the PHY related parts of the power
well programming since we know this situation can only happen with
combo PHYs.

Another option might be to go back to always picking an AUX CH for
all encoders. However I'm a bit wary about that since we might in
theory end up conflicting with the VBT AUX CH assignment. Also
that wouldn't help with encoders not declared in the VBT, should
we ever need to poke the corresponding power wells.

Longer term we need to figure out what the actual relationship
is between the PHY vs. AUX CH vs. AUX power well. Currently this
is entirely unclear.

Cc: stable@vger.kernel.org
Fixes: 9856308c

 ("drm/i915: Only populate aux_ch if really needed")
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/10184
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240223203216.15210-1-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>