drm/i915: Use offsetof() to calculate the offset of members in PVINFO page

To get the offset of the members in PVINFO page, offsetof() looks much
better than the tricky approach in current code.

v7:

- Move "offsetof()" modification into a dedicated patch. (Joonas)

Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
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/1466078825-6662-3-git-send-email-zhi.a.wang@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_pvinfo.h

@@ -104,7 +104,7 @@ struct vgt_if {
 } __packed;
 
 #define vgtif_reg(x) \
-	_MMIO((VGT_PVINFO_PAGE + (long)&((struct vgt_if *)NULL)->x))
+	_MMIO((VGT_PVINFO_PAGE + offsetof(struct vgt_if, x)))
 
 /* vGPU display status to be used by the host side */
 #define VGT_DRV_DISPLAY_NOT_READY 0