drm/i915: Store max lane count in intel_digital_port

Rather than having open coded checks for the DDI A/E configuration,
just store the max supported lane count in intel_digital_port.

We had an open coded check for DDI A, but not for DDI E. So we may
have been vilating the DDI E max lane count.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>