drm/i915/dsi: abstract dphy parameter init

intel_dsi_vbt_init() has grown too unwieldy, and it's about to be
modified due to ICL DSI. Abstract out the VLV specific dphy param
init. No functional changes. Intentionally no stylistic changes during
code movement.

Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/96d15760db027a137f298ec330520ef8ec6474b0.1539613303.git.jani.nikula@intel.com