drm/atomic: Loosen FB atomic checks

Loosen the requirements for atomic and legacy commit so that, in cases
where pixel_source != FB, the commit can still go through.

This includes adding framebuffer NULL checks in other areas to account for
FB being NULL when non-FB pixel sources are enabled.

To disable a plane, the pixel_source must be NONE or the FB must be NULL
if pixel_source == FB.

Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231027-solid-fill-v7-7-780188bfa7b2@quicinc.com