drm/i915: Fix not finding the VBT when it overlaps with OPREGION_ASLE_EXT

drm/i915: Fix not finding the VBT when it overlaps with OPREGION_ASLE_EXT

BugLink: http://bugs.launchpad.net/bugs/1673118
commit 998d75730b40afc218c059d811869abe9676b305 upstream.

If there is no OPREGION_ASLE_EXT then a VBT stored in mailbox #4 may
use the ASLE_EXT parts of the opregion. Adjust the vbt_size calculation
for a vbt in mailbox #4 for this.

This fixes the driver not finding the VBT on a jumper ezpad mini3
cherrytrail tablet and on a ACER SW5_017 machine.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487088758-30050-1-git-send-email-jani.nikula@intel.com
(cherry picked from commit dfb65e71ea2c1d97ac373cc0587dc60b3307581a)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>