drm/amdgpu/smu: Add message sending lock

This adds a message lock to the smu_send_smc_msg* implementations to
protect against concurrent access to the mmu registers used to
communicate with the SMU

v2: Implement for smu_v12_0 as well

v3: Add mutex_init for message_lock

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