drm/i915/display: Implement new combo phy initialization step

This is new step that was recently added to the combo phy
initialization.

v2:
- using intel_de_rmw()

v3:
- going back to read() modify and write() as group register can't be
read

BSpec: 49291
Cc: Clinton A Taylor <clinton.a.taylor@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200625195252.39312-1-jose.souza@intel.com