drm/xe/uapi: Change rsvd to pad in struct drm_xe_class_instance

drm/xe/uapi: Change rsvd to pad in struct drm_xe_class_instance

Change rsvd to pad in struct drm_xe_class_instance to prevent the field
from being used in future.

v2: Change from fixup to regular commit because this touches the
    uAPI (Francois Dugast)

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>