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