drm/i915/tc: Use the cached max lane count value

drm/i915/tc: Use the cached max lane count value

Use the PHY's cached max lane count value on all platforms similarly to
LNL+. On LNL+ using the cached value is mandatory - since the
corresponding HW register field can get cleared by the time the value is
queried - on earlier platforms there isn't a problem with using the HW
register instead. Having a uniform way to query the value still makes
sense and it's also a bit more efficient, so do that.

Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250805073700.642107-7-imre.deak@intel.com
Signed-off-by: Imre Deak <imre.deak@intel.com>