libdrm: Fix some warnings reported by clang's scan-build tool

[Marcin Slusarz]

On Fri, Jul 13, 2012 at 05:49:12PM +0200, Johannes Obermayr wrote:
> 
> Patches 1 to 4 were sent to mesa-dev.

And you chose to ignore most of my comments.
Fine. Don't expect further reviews from me.

Marcin