drm/xe: Wait for success on guc done.

Rather than a constant check on proto and wait not busy,
let's wait for the expected success and then check the
protocol afterwards.

With this, we can now use the regular xe_mmio_wait32
and kill this local need for the wait_for.

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