compositor-drm: Clean up page_flip_pending path

compositor-drm: Clean up page_flip_pending path

page_flip_pending is only be set when do a pageflip to a newly-rendered
buffer; if the flag is not set, we have landed in the start_repaint_loop
path where the vblank query fails, and thus we must pageflip to the same
buffer.

This test was not sufficient for what it was supposed to guard:
releasing framebuffers back. When using client-supplied framebuffers, it
is possible to reuse the same buffer multiple times, and we would send a
framebuffer-release event too early.

However, since we have a properly reference-counted drm_fb now, we can
just drop this test, and rely on the reference counting to prevent
too-early release of client framebuffers.

page_flip_pending now becomes exactly what the name suggests: a flag
which indicates whether or not we are expecting a pageflip event. Add
asserts here to verify that we never receive a pageflip event we weren't
expecting.

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