drm/i915/ddi: Get AUX power domain for DP main link too

So far we got an AUX power domain reference only for the duration of DP
AUX transfers. However, the following suggests that we also need these
for main link functionality:
- The specification doesn't state whether it's needed or not for main
  link functionality, but suggests that these power wells need to be
  enabled already during display core initialization (Sequences to
  Initialize Display).
- For PSR we need to keep the AUX power well enabled.
- On ICL combo PHY ports (non-TC) the AUX power well is needed for
  link training too: while the port is enabled with a DP link training
  test pattern trying to toggle the AUX power well will time out.
- On ICL MG PHY ports (TC) the AUX power well is needed also for main
  link functionality (both in DP and HDMI modes).
- Windows enables these power wells both for main and AUX lane
  functionality.

Based on the above take an AUX power reference for main link
functionality too. This makes a difference only on GEN10+ (GLK+)
platforms, where we have separate port specific AUX power wells.

For PSR we still need to distinguish between port A and the other
ports, since on port A DC states must stay enabled for main link
functionality, but DC states must be disabled for driver initiated
AUX transfers. So re-use the corresponding helper from intel_psr.c.

Since we take now a reference for main link functionality on all DP
ports we can forgo taking the separate power ref for PSR functionality.

v2:
- Make sure DC states stay enabled when taking the ref on port A.
  (Ville)

v3: (Ville)
- Fix comment about logic for encoders without a crtc state and
  add FIXME note for a simplification to avoid calling get_power_domains
  in such cases.
- Use intel_crtc_has_dp_encoder() instead !intel_crtc_has_type(HDMI).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
[Clarified code comments in intel_ddi_main_link_aux_domain() and
 intel_ddi_get_power_domains() (Imre)]
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180621184449.26634-1-imre.deak@intel.com