compositor-drm: Remove crtc_allocator

compositor-drm: Remove crtc_allocator

crtc_allocator was used as a bitmask of CRTC IDs, so we didn't try to
use the same CRTC for multiple outputs. Unfortunately, this only works
to the extent that CRTC object IDs fit within the bitmask; though they
were previously, they are not guaranteed to be under 32 or even 64.

Replace the only use of crtc_allocator with a list walk across outputs.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reported-by: Peter Senna Tschudin <peter.senna@collabora.com>