drm/i915: consider the source max DP lane count too

Even if the panel claims it can support 4 lanes, there's the
possibility that the HW can't, so consider this while selecting the
max lane count.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>