drm/virtio: Support partial maps of GEM objects

drm/virtio: Support partial maps of GEM objects

Those are useful to implement coherent cross-vm mmap.

Signed-off-by: Sasha Finkelstein <fnkl.kernel@gmail.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250109-virtgpu-gem-partial-map-v1-1-a914b48776bd@gmail.com
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
[dmitry.osipenko@collabora.com: Corrected vm_size check to account vm_pgoff]