drm/i915/dsi: add skip functions for spi and pmic elements

In sequence block v3 these are gracefully skipped anyway, but add the
functions so we can have some debug breadcrumbs.

v2: the pmic block is 15 bytes (Ville)

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/d102ef1f21e6ea9a17655ef31593e68343336a48.1474286487.git.jani.nikula@intel.com