Replace the calls into ttm_eu_reserve_buffers with the drm_exec helpers.
Also reuse some code.

v4:
- Kerneldoc xe_vm_prepare_vma().
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230908091716.36984-6-thomas.hellstrom@linux.intel.comSigned-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>