rust: drm: gem: Add raw_dma_resv() function

rust: drm: gem: Add raw_dma_resv() function

For retrieving a pointer to the struct dma_resv for a given GEM object. We
also introduce it in a new trait, BaseObjectPrivate, which we automatically
implement for all gem objects and don't expose to users outside of the
crate.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Janne Grunau <j@jananu.net>
Tested-by: Janne Grunau <j@jannau.net>
Tested-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Link: https://patch.msgid.link/20260316211646.650074-3-lyude@redhat.com
[ Fix incorrect reference in safety comment. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>