xserver master warning fixes

[Eric Anholt]

Here's a series of compile warning fixes I came up with while looking at them
trying to figure out if anything was wrong that broke GLX (fix for that is
already pushed).  The ddxDevBtn.c one looks like a real bugfix.

There are a few more that look important
xf86Xinput.c:654: warning: ‘idev’ may be used uninitialized in this function
xf86Xinput.c:653: warning: ‘drv’ may be used uninitialized in this function
I couldn't convince myself that this was just the compiler getting it wrong.

xf86Helper.c:2727: warning: passing argument 2 of ‘GetMotionHistory’ from incompatible pointer type
This function looks completely broken now -- consumers of this API used to pass
in storage, but now the function wants to do it itself, and this helper func
doesn't get it.