compositor: remove repaint_data from compositor

The repaint_data is entirely backend specific. Moreover, it is only used by the
drm backend, while other backends ignore the repaint data.

There will always be only one repaint active, thus, there is no need to pass the
repaint data from the outside.

The repaint_data breaks with the multi-backend series, which calls repaint begin
for all backends to get the repaint_data. The repaint_data of the last backend
will then be passed to all other backend. At the moment, this works, because the
drm backend is the only backend that implements the begin_repaint call.

Another option would be to track the repaint data per backend in the compositor,
but actually, it the backend needs to track state across the calls, it's its own
responsibility.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>