drm/i915/dp: abstract dp link config computation from the rest

Abstract a new intel_dp_compute_link_config() from
intel_dp_compute_config(), with the parts related to link configuration,
i.e. bpp, link rate, and lane count selection. No functional changes.

v2: Fix a checkpatch warn about spacing.

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/80f99a625633f87f44d38d487ba3b32ff9a26b07.1524730974.git.jani.nikula@intel.com