drm/i915: Add state verification for the TypeC port mode

Add state verification for the TypeC port mode wrt. the port's AUX power
well enabling/disabling. Also check the correctness of changing the port
mode:
- When enabling/disabling the AUX power well for a TypeC port we must hold
  the TypeC port lock - the case for AUX transfers - or hold a Type C
  port link reference - the case for modeset enabling/disabling.
- When changing the TypeC port mode the port's AUX power domain must be
  disabled.

v2: (Ville)
- Simplify power_well_async_ref_count().
- Fix the commit log, clarifying what are the valid conditions to
  enable/disable the AUX power wells.

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@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/20190628143635.22066-22-imre.deak@intel.com