compositor-drm: Avoid cast by using unsigned loop index

ARRAY_LENGTH returns a size_t; rather than casting its result to
int so we can compare to our signed index variable, just declare the
index as a compatible type in the first place.

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