drm/xe/uapi: Separate bo_create placement from flags

Although the flags are about the creation, the memory placement
of the BO deserves a proper dedicated field in the uapi.

Besides getting more clear, it also allows to remove the
'magic' shifts from the flags that was a concern during the
uapi reviews.

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>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>