drm/i915: Factor out i915_pvinfo.h

As the PVINFO page definition is used by both GVT-g guest (vGPU) and GVT-g
host (GVT-g kernel device model), factor it out for better code structure.

v7:
- Split the "offsetof" modification into a dedicated patch. (Joonas)

v3:
- Use offsetof to calculate the member offset of PVINFO structure (Joonas)

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466078825-6662-2-git-send-email-zhi.a.wang@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>