drm/xe/mcr: Use designated init for xe_steering_types

There is already a BUILD_BUG_ON() check to make sure the size follow the
number of steering types. Also make sure the right index is being used
for each steering type.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>