There's a comment explaining how to hack the DRM-backend in order to
fake that a certain format is not supported by the KMS device. This is
useful in order to test dma-buf feedback implementations using the
simple-dmabuf-feedback client. But with recent changes on the
DRM-backend, this got outdated.

Drop this comment, as everyone interested in this client is probably
familiar enough with the DRM-backend in order to do that.

Also made some adjustments to other comments explaining how this client
works.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>