drm/i915/dsi: add debug logging to element execution

Just simple breadcrumbs for now. While at it, rename the i2c skip
function.

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/d11c40a99f5ef2419ede87a2ac1858e4c60768b8.1474286487.git.jani.nikula@intel.com