drm/xe: skip eviction of a BO with no resource

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

drm/xe: skip eviction of a BO with no resource

xe_evict_flags() switches on tbo->resource->mem_type to choose an
eviction placement. An xe BO can reach eviction with no ttm.resource,
e.g. an imported dma-buf BO whose resource was freed during teardown
while its attachment is still live: the exporter then invalidates it via
xe_dma_buf_move_notify() -> xe_bo_evict(), and the switch dereferences a
NULL resource and faults.

A BO with no resource has nothing placed or mapped, so there is nothing
to evict. Return an empty placement (num_placement = 0), mirroring the
handling already used for non-xe scatter-gather BOs just above;
ttm_bo_validate() then takes the pipeline-gutting path and returns 0.

Without the fix the xe_dma_buf@xe_test_dmabuf_move_notify_no_resource
dynamic live selftest will fail with below stack.

BUG: kernel NULL pointer dereference, address: 0000000000000010
RIP: 0010:xe_evict_flags+0xf4/0x140 [xe]
Call Trace:
<TASK>
xe_bo_evict+0x40/0xa0 [xe]
? ttm_bo_pipeline_gutting+0x71/0x180 [ttm]
xe_test_dmabuf_move_notify_no_resource+0x182/0x440 [xe]
xe_dma_buf_kunit+0x387/0x580 [xe]

v2: Add Fixes tag and failure info in commit text (Matt Brost)

(cherry picked from commit dbcfaac6331f1fcb0c2fe3ad3e9adb1e5d43ec0c)