backend-drm: Don't try to prepare a non-cursor view in renderer-only mode

Doing this might cause unnecessary DRM importing, which might result in
an error when the DRM's memory address mapping is nearly full.

Return before attempting to create drm_fb to avoid that, since it will
fail in the later check anyway.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>