drm/gem/shmem: Extract drm_gem_shmem_init() from drm_gem_shmem_create()

drm/gem/shmem: Extract drm_gem_shmem_init() from drm_gem_shmem_create()

With gem objects in rust, the most ideal way for us to be able to handle
gem shmem object creation is to be able to handle the memory allocation of
a gem object ourselves - and then have the DRM gem shmem helpers initialize
the object we've allocated afterwards. So, let's split out
drm_gem_shmem_init() from drm_gem_shmem_create() to allow for doing this.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20250911230147.650077-2-lyude@redhat.com