shaders/colorspace: don't warn on solid black frame

Since 31082eee it's possible for frame_wg_count to not get incremented, in particular, for all black WGs. This may lead to the scenario where frame_wg_count is 0 for the frame/slice. Avoid false positive on the allow_delayed check and also handle this case correctly by introducing explicit frame_ready field which is set to 1 once per compute shader invocation.

Fixes: 31082eee