drm/i915/dp: group link config limits in a struct

Also use same min/max model for bpp, and adjust debug logging while at
it.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/72f78c7ae0cd1810798bd94cbf5e574c78da83f8.1524730974.git.jani.nikula@intel.com