automated/linux/piglit: Make usage of XvFB optional

Piglit tests can fail when use framebuffer to execute, so don't use
xvfb by default.

Change-Id: Idd7d7d9d496084424e44e35c81f522233d6a551c
Signed-off-by: Aníbal Limón <anibal.limon@linaro.org>
1 file changed