drm/i915/dsi: skip unknown elements for sequence block v3+

The sequence block has sizes of elements after the operation byte since
sequence block v3. Use it to skip elements we don't support yet.

v2: remove redundant exec_elem[operation_byte] check (Daniel)

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452006408-27688-1-git-send-email-jani.nikula@intel.com