drm/i915/bios: abstract finding the panel sequence block

Make the whole thing easier to read. While at it, make the parsing more
robust, and ensure we don't read past buffer being parsed.

v2: improve commit message (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/1452001851-8967-1-git-send-email-jani.nikula@intel.com