drm/i915/bios: have functions return vbt, not bdb, header pointer

This will simplify further work.

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/d2c5210402fdd8c277e1d50892b0620d10c50ae8.1450089383.git.jani.nikula@intel.com