drm/amdgpu/powerplay: Refactor SMU message handling for safety

Move the responsibility for reading argument registers into the
smu_send_smc_msg* implementations, so that adding a message-sending lock
to protect the SMU registers will result in the lock still being held
when the argument is read.

v2: transition smu_v12_0, it's asics, and vega20

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