drm/i915/dsi: refactor dsi get hw state readout

Make the code easier to read and update. No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/04b20a9be6a9481ca2eb2a42cdcdc27358152e5f.1458070700.git.jani.nikula@intel.com