drm: vkms: get the reference to `drm_framebuffer` instead if coping it

Instead of coping `drm_framebuffer` - which can cause problems -
we just get the reference and add the ref count.

Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Igor Torrente <igormtorrente@gmail.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220905190811.25024-5-igormtorrente@gmail.com