drm_atomic: do not set immutable properties

On some platforms the ZPOS property might exist, but be immutable.
This is at least the case on Intel Sandy Bridge since Linux kernel
5.5.0. Trying to set an immutable property will cause.
drmModeAtomicCommit to fail with -EINVAL.

On other platforms we might want to set ZPOS to tweak the layering of
planes.

To reconcile these two, simply have drm_object_set_property check if a
property is immutable before attempting to add it to the atomic
commit, instead returning an error code (which is, as previously,
ignored in the case of ZPOS as we don't strictly need it)