backend-drm: Fix count page flips on invalid outputs

Slight oversight of counting page flips from within the atomic commit
handler when we don't really have an output. Do that after *after*
checking for a valid output.

Fixes: cda8de10, ("backend-drm: Count and display KMS page flips")

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>