drm/i915: Allow per-lane drive settings with LTTPRs

LTTPRs should support per-lane drive settings I think, and even if
they don't they should implement their own fallback logic to determine
suitable common drive settings to use for all the lanes.

v2: Actually check the correct thing

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211001130107.1746-11-ville.syrjala@linux.intel.com