drm/shmem-helper: Add lockdep asserts to vmap/vunmap

drm/shmem-helper: Add lockdep asserts to vmap/vunmap

Since commit 21aa27ddc582 ("drm/shmem-helper: Switch to reservation
lock"), the drm_gem_shmem_vmap and drm_gem_shmem_vunmap functions
require that the caller holds the DMA reservation lock for the object.
Add lockdep assertions to help validate this.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://lore.kernel.org/r/20250318-drm-gem-shmem-v1-1-64b96511a84f@collabora.com