gpu/stub: fix acpi_video build error, fix stub kconfig dependencies

The comments under "config STUB_POULSBO" are close to correct,
but they are not being followed.  This patch updates them to reflect
the requirements for THERMAL.

This build error is caused by STUB_POULSBO selecting ACPI_VIDEO
when ACPI_VIDEO's config requirements are not met.

ERROR: "thermal_cooling_device_register" [drivers/acpi/video.ko] undefined!
ERROR: "thermal_cooling_device_unregister" [drivers/acpi/video.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Dave Airlie <airlied@redhat.com>