drm/amdgpu: cleanup ctx implementation

Let each context have a pointer to the ctx manager and properly
initialize the adev pointer inside the context manager.

Reduce the BUG_ON() in amdgpu_ctx_add_fence() into a WARN_ON() and
directly return the sequence number instead of writing into a parmeter.

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