drm/i915: hide away VBT private data in a separate header

We've been accumulating code across the driver that depends on the VBT
specific structures and defines. The VBT is an uncontrollable
beast. Encourage encapsulation of the VBT data by hiding the structures
and defines in a private header only to be included from intel_bios.c.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458125015-7931-7-git-send-email-jani.nikula@intel.com