drm/gpuvm: drm_gpuvm_bo_obtain() requires lock and staged mode

drm/gpuvm: drm_gpuvm_bo_obtain() requires lock and staged mode

In commit 9ce4aef9a5b1 ("drm/gpuvm: take GEM lock inside
drm_gpuvm_bo_obtain_prealloc()") we update
drm_gpuvm_bo_obtain_prealloc() to take locks internally, which means
that it's only usable in immediate mode.

In this commit, we notice that drm_gpuvm_bo_obtain() requires you to use
staged mode. This means that we now have one variant of obtain for each
mode you might use gpuvm in.

To reflect this information, we add a warning about using it in
immediate mode, and to make the distinction clearer we rename the method
with a _locked() suffix so that it's clear that it requires the caller
to take the locks.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260108-gpuvm-rust-v2-2-dbd014005a0b@google.com
[ Slightly reword commit message to refer to commit 9ce4aef9a5b1
  ("drm/gpuvm: take GEM lock inside drm_gpuvm_bo_obtain_prealloc()").
  - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>