backend-drm: skip building atomic state and logging for "empty" repaints

With multiple backends or devices repaint_begin/repaint_flush may be called even
if no outout of a device will be repainted. This results in an "empty" drm state
without any output.
The actual commit to the kernel is already skipped but the drm-backend log is
still filled with "Beginning repaint"/"repaint-flush" messages and the scene
graph.

Use the new prepare_repaint() callback to determine if a backend needs to be
repainted and only create the pending_state if necessary.
Exit early in repaint_flush()/repaint_cancel() when no pending_state was created.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>