drm/gem-vram: Share code between GEM VRAM's _{prepare, cleanup}_fb()

The error-recovery code in drm_gem_vram_plane_helper_prepare_fb() is of
the same pattern as drm_gem_vram_plane_helper_cleanup_fb(). Implement
both of them using an internal helper. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220517113327.26919-4-tzimmermann@suse.de