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

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.

Cc: [email protected]
Signed-off-by: Hans de Goede <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]