drm/xe/tests/xe_dma_buf: Set the drm_object::dma_buf member

This member is set when exporting using prime. However
the xe_gem_prime_export() alone doesn't set it, since it's done
later in the prime export flow.
For the test, set it manually and remove the hack that set it
temporarily when it was really needed.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250821143045.106005-3-thomas.hellstrom@linux.intel.com