drm/i915/dp: add functions for max common link rate and lane count

These are the theoretical maximums common for source and sink. These are
the maximums we should start with. They may be degraded in case of link
training failures, and the dynamic link values are stored separately.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/5088aca253c47dfa18251e1adb976aca1718f083.1491485983.git.jani.nikula@intel.com