gbm: Removed unused function.

AFAICT, it's never been used.

It was briefly nudged in the right direction here:
commit 10e5ffd4961055ebba5be4d85a93cc66cdd5a635
Author: Emil Velikov <emil.l.velikov@gmail.com>
Date:   Sat Jan 25 17:19:10 2014 +0000

    gbm: do not export _gbm_mesa_get_device

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
2 files changed