drm/i915: Parse max link rate from the eDP BDB block

The eDP BDB block has gained yet another max link rate field.
Let's parse it and consult it during the source rate filtering.

v2: *20 instead of *2 to get the correct units (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220602205723.11341-1-ville.syrjala@linux.intel.com