drm/i915/debugfs: add a separate debugfs file for VBT

In the future the VBT might not be in mailbox #4 of the ACPI OpRegion,
thus unavailable in i915_opregion, so add a separate file for the VBT.

v2: Drop the locking as unneeded (Chris)
v3: Rebase

Cc: Chris Wilson <chris@chris-wilson.co.uk>
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/1450178232-27780-1-git-send-email-jani.nikula@intel.com
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 10e4716..ca8c2a6 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -458,6 +458,7 @@
 	u32 swsci_sbcb_sub_functions;
 	struct opregion_asle *asle;
 	const void *vbt;
+	u32 vbt_size;
 	u32 *lid_state;
 	struct work_struct asle_work;
 };