drm/amdgpu/smu: add metrics table lock

This table is used for lots of things, add it's own lock.

Bug: https://gitlab.freedesktop.org/drm/amd/issues/900


Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org