drm/atomic: Stable sort for atomic request de-duplication

Atomic request property lists are defined to be de-duplicated: an atomic
request can contain multiple sets for the same property on the same
object, and only the last one will take effect.

drmModeAtomicCommit already sorts the property set by object and
property ID. We were relying on qsort to also sort by cursor - i.e.
pointer value - when object and property ID are equal, however whilst
glibc does this, the sort order is explicitly undefined when the
comparator is equal. Using the pointer is also not stable on all
implementations.

Add an explicit 'cursor' member to each property set which is used as
the tie-breaker comparator.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Fixes: #46