backend-drm: fix possible leak of struct drm_output

Before this patch, we would leak the drm_output if there was a pending
flip during shutdown.

Now we destroy the drm_output even if there's a pending flip (only
during shutdown, as we don't want to wait until flip completion to
destroy the output).

Also, it fixes a problem where weston_output_enable() is called right
after weston_output_enable() or weston_output_disable() and it could
fail to find available DRM objects (as they are only released after
the flip completion).

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