drm/amdgpu: Make SDMA phase quantum configurable

Set a configurable SDMA phase quantum when enabling SDMA context
switching. The default value significantly reduces SDMA latency
in page table updates when user-mode SDMA queues have concurrent
activity, compared to the initial HW setting.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Andres Rodriguez <andres.rodriguez@amd.com>
Reviewed-by: Shaoyun Liu <shaoyun.liu@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>