drm/amd/pm: Disable ULV even if unsupported (v3)

Always send PPSMC_MSG_DisableULV to the SMC, even if ULV mode
is unsupported, to make sure it is properly turned off.

v3:
Simplify si_disable_ulv further.
Always check the return value of amdgpu_si_send_msg_to_smc.

Fixes: 841686df ("drm/amdgpu: add SI DPM support (v4)")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>