dix: add dixAnyOtherGrabbed() and unexport GrabInProgress

[metux]

• the GrabInProgress field isn't used by any known drivers, thus no need to export it
• the only in-tree consumer outside of it's source file is Xext/saver.c, which actually just wants to know whether any other client already grabbed the server - asking the OS layer isn't the right source (should ask dix/dispatch.c instead)

Therefore adding a simple function for that check into dispatch.c (where grab/ungrab is actually controlled), so we can easily make this internal field static again.

Signed-off-by: Enrico Weigelt, metux IT consult [email protected]