drm/gpuvm: take GEM lock inside drm_gpuvm_bo_obtain_prealloc()

drm/gpuvm: take GEM lock inside drm_gpuvm_bo_obtain_prealloc()

When calling drm_gpuvm_bo_obtain_prealloc() and using immediate mode,
this may result in a call to ops->vm_bo_free(vm_bo) while holding the
GEMs gpuva mutex. This is a problem if ops->vm_bo_free(vm_bo) performs
any operations that are not safe in the fence signalling critical path,
and it turns out that Panthor (the only current user of the method)
calls drm_gem_shmem_unpin() which takes a resv lock internally.

This constitutes both a violation of signalling safety and lock
inversion. To fix this, we modify the method to internally take the GEMs
gpuva mutex so that the mutex can be unlocked before freeing the
preallocated vm_bo.

Note that this modification introduces a requirement that the driver
uses immediate mode to call drm_gpuvm_bo_obtain_prealloc() as it would
otherwise take the wrong lock.

Fixes: 63e919a31625 ("panthor: use drm_gpuva_unlink_defer()")
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260108-gpuvm-rust-v2-1-dbd014005a0b@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>