drm/amdgpu/psp: flush HDP write fifo after submitting cmds to the psp

We need to make sure the fifo is flushed before we ask the psp to
process the commands.

Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>