drm/i915/gen9_bc: Add W/A for missing STRAP config on TGP PCH + CML combos

Apparently the new gen9_bc platforms that Intel has introduced don't
provide us with a STRAP config register to read from for initializing DDI
B, C, and D detection. So, workaround this by hard-coding our strap config
in intel_setup_outputs().

Changes since v4:
* Split this into it's own commit

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
[originally from Tejas's work]
Signed-off-by: Tejas Upadhyay <tejaskumarx.surendrakumar.upadhyay@intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209212832.1401815-5-lyude@redhat.com