drm/xe: Fix DPT allocation paths.

In the Linux kernel, the following vulnerability has been resolved:

drm/xe: Fix DPT allocation paths.

Remove the fallback for VRAM to system memory, I tested it and that
doesn't work at all, only a black screen with pipe fault errors were
observed.

On systems with media GT, extra latency is added when accessing stolen
memory when the GT is in MC6. Since we additionally aren't counting how
much memory is used for stolen and we could in theory fill up the
entire stolen area with DPT's, avoid using stolen and only use the
default memory region.

Using stolen may also result in random system hangs under load.

(cherry picked from commit a196406a3831291598fe8e73245914f7acffdfe0)