Fix DGA events

[Ville Syrjala]

This series tries to fix DGA input support.

Note that even after these patches, DGA2 seems to be completely
useless. XDGASelectInput is refused unless XDGASetMode is done first.
However XDGASetMode disables all X rendering, and since the other
DGA2 rendering paths (XDGAOpenFrameBuffer, pixmap, XDGAFillRect & co.)
were disabled, there is absolutely no way to put any content on the
screen while in DGA2 mode.