drm/amdgpu/soc15: don't abuse IP soft reset for adapter reset

The IP soft reset interface is for per IP reset but it was
being abused for adapter reset on soc15 asics.  Adjust the
interface to make it explicit.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>