compositor-drm: Return FB directly from render

compositor-drm: Return FB directly from render

Instead of setting state members directly in the drm_output_render
functions (to paint using Pixman or GL), just return a drm_fb, and let
the core function place it in state.

This brings damage handling in line with repaint state, so we do not
clear damage if repaint fails.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>