drm/i915/bios: split the MIPI DSI VBT block parsing to two

There's two blocks to parse, have one function per block. The existing
one cuts neatly into two.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/6c9598e2b4d07e8d264617cdfe8b6527a74261f7.1450702954.git.jani.nikula@intel.com