drm/amdkfd: Replace cpu_relax() with schedule() in DQM

In order not to occupy the current core and thus prevent the core from
servicing IOMMU PPR requests, this patch replaces the call in DQM to
cpu_relax() with a call to schedule().

Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>