drm/i915/dsi: abstract intel_dsi_tlpx_ns()

Will be needed in the future. No functional changes.

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/2cb427e5bc2ea88e4226bfcf162b3a6f307e32e1.1539613303.git.jani.nikula@intel.com