drm/i915: Update eDP fast link training link rate parsing

drm/i915: Update eDP fast link training link rate parsing

We're not parsing the 5.4 Gbps value for the old eDP fast link
training link rate, nor are we parsing the new fast link training
link rate field. Remedy both.

Also we'll now use the actual link rate instead of the DPCD BW
register value.

Note that we're not even using this information for anything
currently, so should perhaps just nuke it all unless someone
is planning on implementing fast link training finally...

v2: Stop using the DPCD BW values (Jani)
    *20 instead of *2 to get the rate in correct units (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220602205649.11283-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>