drm/amdkfd: Reduce priority of context-saving waves before spin-wait

Synchronization between context-saving wavefronts is achieved by
sending a SAVEWAVE message to the SPI and then spin-waiting for a
response. These spin-waiting wavefronts may inhibit the progress
of other wavefronts in the context save handler, leading to the
synchronization condition never being achieved.

Before spin-waiting reduce the priority of each wavefront to
guarantee foward progress in the others.

Signed-off-by: Jay Cornwall <Jay.Cornwall@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>