drm/shmem-helper: Unexport drm_gem_shmem_create_with_handle()

Turn drm_gem_shmem_create_with_handle() into an internal helper
function. It's not used outside of the compilation unit.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211108093149.7226-2-tzimmermann@suse.de