drm/amdgpu/powerplay/vega10: fix memory leak in error path

Free the backend structure if we fail to allocate device
memory.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>