drm/i915/dp: Ensure sink/link max lane count values are always valid

Print an error if the DPCD sink max lane count is invalid and fix it up.

While at it also add an assert that the link max lane count (derived
from intel_dp_max_common_lane_count(), potentially reduced by the LT
fallback logic) value is also valid.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018094154.1407705-5-imre.deak@intel.com