drm/amd/pm: Fix si_upload_smc_data (v3)

The si_upload_smc_data function uses si_write_smc_soft_register
to set some register values in the SMC, and expects the result
to be PPSMC_Result_OK which is 1.

The PPSMC_Result_OK / PPSMC_Result_Failed values are used for
checking the result of a command sent to the SMC.
However, the si_write_smc_soft_register actually doesn't send
any commands to the SMC and returns zero on success,
so this check was incorrect.

Fix that by not checking the return value, just like other
calls to si_write_smc_soft_register.

v3:
Additionally, when no display is plugged in, there is no need
to restrict MCLK switching, so program the registers to zero.

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>