drm/i915: Organize VBT stuff inside drm_i915_private

drm_i915_private is getting bigger and bigger when adding new vbt stuff.
So, the better way of getting drm_i915_private organized is to create
a special structure for vbt stuff.

v2: Basically conflicts fixes

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>