drm/i915: Use the AUX_IO power domain only for eDP/PSR port

Use the AUX_IO_A display power domain only for eDP on port A where PSR
is also supported. This is the case where DC states need to be enabled
while the output is enabled - ensured by AUX_IO_A domain not enabling
the DC_OFF power well. Otherwise port A can be treated the same way as
other ports with an external DP output: using the AUX_<port> domain
which disables the unrequired DC states.

This change prepares for the next patch enabling DC states on all ports
supporting eDP/PSR besides port A.

v2:
- Check the encoder PSR capability instead of PSR being enabled in the
  crtc_state, as the latter can be changed with a fastset.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221114122251.21327-5-imre.deak@intel.com