drm/i915/dp: fix DG1 and RKL max source rates

Combo phy is limited to 5.4 GHz on low-voltage SKUs, but both eDP and DP
can do 8.1 GHz on combo phy.

Bspec: 49182, 49205, 49202

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210901160402.24816-5-animesh.manna@intel.com