drm/ttm/xe: drop unused force_alloc flag

This flag used to be used in the old memory tracking code, that
code got migrated into the vmwgfx driver[1], and then got removed
from the tree[2], but this piece got left behind.

[1] f07069da ("drm/ttm: move memory accounting into vmwgfx v4")
[2] 8aadeb8a ("drm/vmwgfx: Remove the dedicated memory accounting")

Cleanup the dead code.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>