drm/i915/dg2: Update modeset sequences

DG2 has some changes to the expected modesetting sequences when compared
to gen12.  Adjust our driver logic accordingly.  Although the DP
sequence is pretty similar to TGL's, there are some steps that change,
so let's split the handling for that out into a separate function.

v2:
 - Switch wait_for_us() -> _wait_for() so that we can parameterize the
   timeout rather than duplicating the macro call.  (Jani)

Bspec: 54128
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723174239.1551352-27-matthew.d.roper@intel.com