drm/amdgpu: fix gfx9 soft recovery

The SOC15_REG_OFFSET() macro wasn't used, making the soft recovery fail.

v2: use WREG32_SOC15 instead of WREG32 + SOC15_REG_OFFSET

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org