drm/i915/dp: abstract link config selection

For now, there's just the one link config selection, optimizing for slow
and wide link. No functional changes.

Keep the debug logging in the caller, to avoid duplication later on if
alternative link confing selection gets added.

v2: Improved commit message

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/64848b76bf90d6ceecd7ec6b5add28531e0b1a41.1524730974.git.jani.nikula@intel.com