drm/amdgpu/gmc10: program the smallK fragment size

Explicitly set the smallk size to 0 (4k).  This is the hw
default, but set it anyway just in case something else
changed it.

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