drm/xe/oa: Zero OA buffer at allocation

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

drm/xe/oa: Zero OA buffer at allocation

ttm_bo_type_kernel BOs are not guaranteed to be zeroed on allocation and
may contain data from previously freed pages. Since the OA buffer is
mmap'able by userspace, this can expose stale kernel memory contents.

Streams opened with DRM_XE_OA_PROPERTY_OA_DISABLED allocate the OA
buffer without calling xe_oa_init_oa_buffer(). Zero the buffer at
allocation to prevent information disclosure through mmap.

The existing zeroing in xe_oa_init_oa_buffer() remains necessary for
zero report-id/timestamp detection on enable/re-enable.

(cherry picked from commit 126c4c5388f9311a390b9b254a835b0cf0f55331)