drm/xe: Convert xe_vma_op_flags to BIT macros

Rather than open code the shift for values, use BIT macros.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>