compositor-drm: Don't test render-only atomic configuration

In the RENDERER_ONLY state proposal mode, we don't actually have a
viable configuration to test, because we won't get a renderer buffer
until after assign_planes - where we're called from - has completed.

This can result in us trying to test a configuration with the CRTC and
connectors active, but no planes active, which the kernel can
legitimately fail.

If we're working in renderer-only mode, just return the state we have
without trying to test it first, and let the kernel fill it in later.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derek.foreman.samsung@gmail.com>