drm: allow to skip composition if pending capture is writeback

We have an optimization to skip composition if there's no damage on the
primary plane and we already have a renderer buffer active. But we don't
allow this optimization if there's a pending capture task for the
output. For the renderer-based sources, that is really necessary, but
for the writeback source we should allow this optimization.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>