drm/xe/migrate: prevent infinite recursion

If the buf + offset is not aligned to XE_CAHELINE_BYTES we fallback to
using a bounce buffer. However the bounce buffer here is allocated on
the stack, and the only alignment requirement here is that it's
naturally aligned to u8, and not XE_CACHELINE_BYTES. If the bounce
buffer is also misaligned we then recurse back into the function again,
however the new bounce buffer might also not be aligned, and might never
be until we eventually blow through the stack, as we keep recursing.

Instead of using the stack use kmalloc, which should respect the
power-of-two alignment request here. Fixes a kernel panic when
triggering this path through eudebug.

v2 (Stuart):
 - Add build bug check for power-of-two restriction
 - s/EINVAL/ENOMEM/

Fixes: 270172f6 ("drm/xe: Update xe_ttm_access_memory to use GPU for non-visible access")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Maciej Patelczyk <maciej.patelczyk@intel.com>
Cc: Stuart Summers <stuart.summers@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://lore.kernel.org/r/20250731093807.207572-6-matthew.auld@intel.com