drm/amdkfd: Fix race between scheduler and context restore

The scheduler may raise SQ_WAVE_STATUS.SPI_PRIO via SQ_CMD before
context restore has completed. Restoring SPI_PRIO=0 after this point
may cause context save to fail as the lower priority wavefronts
are not selected for execution among spin-waiting wavefronts.

Leave SPI_PRIO at its SPI-initialized or scheduler-raised value.

v2: Also fix race with exception handler

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>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>