drm/amdgpu: add dce_v6_0_soft_reset() to DCE6

DCE6 was missing soft reset, but it was easily identifiable under radeon.
This should be it, pretty much as it is done under DCE8 and DCE10.

Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>