drm/amdgpu: update outdated comment

drm/amdgpu: update outdated comment

The function amdgpu_amdkfd_gpuvm_import_dmabuf() was split into
import_obj_create() and amdgpu_amdkfd_gpuvm_import_dmabuf_fd() in
commit 0188006d7c79 ("drm/amdkfd: Import DMABufs for interop
through DRM").  import_obj_create() now does the allocation for
the mem variable discussed in the comment.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Felix Kuehling <felix.kuehling@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>