Our TGL CI platforms are running into cases where aux transactions have
failed to complete or declare a timeout well after the timeout limit
that the hardware is supposed to enforce.  From the logs it appears that
these failures arise when aux transactions happen after we've entered
DC6:

  <7> [622.523650] [drm:skl_enable_dc6 [i915]] Enabling DC6
  <7> [622.523685] [drm:gen9_set_dc_state [i915]] Setting DC state from 00 to 02
  ...
  <3> [622.535753] [drm:intel_dp_aux_xfer [i915]] *ERROR* dp aux hw did not signal timeout!
  <3> [622.547745] [drm:intel_dp_aux_xfer [i915]] *ERROR* dp aux hw did not signal timeout!
  <3> [622.559746] [drm:intel_dp_aux_xfer [i915]] *ERROR* dp aux hw did not signal timeout!
  <3> [622.571744] [drm:intel_dp_aux_xfer [i915]] *ERROR* dp aux hw did not signal timeout!
  <3> [622.583743] [drm:intel_dp_aux_xfer [i915]] *ERROR* dp aux hw did not signal timeout!
  <3> [622.583780] [drm:intel_dp_aux_xfer [i915]] *ERROR* dp_aux_ch not done status 0xad400bff
  <7> [622.863725] [drm:drm_dp_dpcd_access] Too many retries, giving up. First error: -110

On TGL AUX B & C are in PG1 (managed by the DMC firmware) rather
than PG3 as they were on ICL, so allowing DC6 means the DMC firmware
might shut off the power wells behind our backs when we're trying to use
them.
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025230623.27829-6-matthew.d.roper@intel.comReviewed-by: Imre Deak <imre.deak@intel.com>