drm/amdgpu: make the CTX ioctl thread-safe

The existing locks were protecting the list, but not the elements.

v2: rename hlock to lock

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>