drm/xe/dmabuf: Make xe_dmabuf_ops static

It is not referenced outside of the xe_dma_buf.c source file.

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240117134048.165425-2-thomas.hellstrom@linux.intel.com


(cherry picked from commit e2dc52f8)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>