drm/xe/uapi: Standardize the FLAG naming and assignment

Only cosmetic things. No functional change on this patch.
Define every flag with (1 << n) and use singular FLAG name.

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