drm/i915/bios: have get_blocksize() support MIPI sequence block v3+

Have get_blocksize() support the special case of MIPI sequence block v3+
which has a separate field for size. Provide and use abstractions for
getting the blocksize given a pointer to the block "envelope",
i.e. pointer to the block id, and given a pointer to the block payload
data.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/e935bd5e119a83dd91214c47e6cd4f6ce8b2a17e.1450702954.git.jani.nikula@intel.com