drm_hwcomposer: Add statistics on flattening to dump message

We need some way to verify that current composition is not failed to
compose, but instead sent to flatten on GPU.

Dump message shows current flattening state. It also displays how
often do compositor switches into flattening.

Signed-off-by: Roman Kovalivskyi <roman.kovalivskyi@globallogic.com>
1 job for flattening-on-client in 46 seconds (queued for 1 second)