drm/xe: Use bookkeep slots for external BO's in exec IOCTL

Fix external BO's dma-resv usage in exec IOCTL using bookkeep slots
rather than write slots. This leaves syncing to user space rather than
the KMD blindly enforcing write semantics on every external BO.

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Kenneth Graunke <kenneth.w.graunke@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reported-by: Simona Vetter <simona.vetter@ffwll.ch>
Closes: drm/xe/kernel#2673


Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911152622.903058-1-matthew.brost@intel.com