drm/i915: Future-proof DDC pin mapping

drm/i915: Future-proof DDC pin mapping

We generally assume future platforms will inherit the behavior of the
most recent platforms, so update our DDC pin mapping defaults to match
how ICP/TGP behave (i.e., pins starting from GMBUS_PIN_1_BXT for combo
PHY's and pins starting from GMBUS_PIN_9_TC1_ICP for TC PHY's).  MCC's
non-standard handling of combo PHY C seems like a platform-specific
quirk that is unlikely to be duplicated on future platforms, so continue
handling it as a special case.

Without this change, future platforms would default to gen4-style pin
mapping which is almost certainly not what we'll want.

Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190918235626.3750-1-matthew.d.roper@intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>