drm/i915/dsi: Add intel_dsi_unprepare() helper

The enable path has an intel_dsi_prepare() helper which prepares various
registers for the mode-set. Move the code undoing this to a new
intel_dsi_unprepare() helper function for better symmetry between the
enable and disable paths. No functional changes.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/cc0baaf04ea74a20031b4b5bb128591dcfa78406.1488273823.git.jani.nikula@intel.com