drm/nouveau: retain device pointer in nvkm_gsp_mem object

Store the struct device pointer used to allocate the DMA buffer in
the nvkm_gsp_mem object.  This allows nvkm_gsp_mem_dtor() to release
the buffer without needing the nvkm_gsp.  This is needed so that
we can retain DMA buffers even after the nvkm_gsp object is deleted.

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030202952.694055-1-ttabi@nvidia.com