clients/simple-dmabuf-egl: Add some notes when using direct-display

clients/simple-dmabuf-egl: Add some notes when using direct-display
protocol

As dmabuf uses a different coordinate (top-left) system than OpenGL
(bottom-left) using both direct-display with the Y_INVERT dmabuf attrib
flag would result in the image being inverted (direct-display will
remove the Y_INVERT flag, which caused the image to be displayed
correctly). Notifies users that direct-display is in use.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>