libdrm: Fix drm.h include in qxl drm header file

Use "drm.h" instead of "drm/drm.h" as used in the other header files.
Fixes xserver-xorg-video-qxl build with KMS support on Debian, where this
file is installed in /usr/include/libdrm.

Fixes Debian bug #746807

Reported-by: Bastian Blank <waldi@debian.org>
Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>