drm/amdgpu/powerplay: Remove deprecated smc_read_arg

The new interface reads the argument in the call to send the message, so
this is no longer needed, and shouldn't be used for concurrency safety
reasons.

Signed-off-by: Matt Coffin <mcoffin13@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>