mesa: Make _mesa_primitive_restart_index a static inline in the header.

mesa: Make _mesa_primitive_restart_index a static inline in the header.

It's now basically a single expression, so it probably makes sense to
have it inlined into the callers.

Suggested by Marek.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>