drm/amdgpu: move i2c bus lock out of ras structure

It's not really ras related.  It's just a lock for the
bus in general.  This removes the ras dependency from
the smu i2c bus.

Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>