drm/amdgpu: Check if SW SMU is supported before accessing funcs

smu.ppt_funcs is only initialized for ASICs supporting SW SMU.

On a Hawaii ASIC, attempting to access the udev attribute
ATTRS{power_dpm_state} will cause a null pointer deref in
amdgpu_get_dpm_state() because of this.

Fix by checking if SW SMU is supported first.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>