drm/amdgpu: enable sw smu driver for navi10 by default

Navi10 will use sw smu driver for dynamic power managment,
while vega20 could also use sw smu driver when amdgpu_dpm is
set to 2

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