randr: stop immutable output properties from being deleted

[Luc Verhaegen]

There are two patches coming up here, one for the server, one for
randrproto.

After having received his Nokia N950, Ville Syrjälä noticed that he hadn't
properly marked all output properties as immutable. Since we are closing in on
selling the nokia N9, and inclusion of code has become extremely difficult, i
needed to prove that this was a real issue, so a quick hack which tries to
delete all properties from all outputs was written. This succeeded,
completely, while it should've only succeeded on those bits that were not
marked immutable in the N9s omap fb driver.

The first patch, for the server, stops immutable properties from getting
deleted by the client.

The second patch, for randrproto,clarifies the situation of deleting output
properties.

Luc Verhaegen.