gbm: remove never-implemented function

I assume this was implemented in a previous version of that commit, but
was removed in the version that actually landed.

Fixes: 8430af5e "Add support for swrast to the DRM EGL platform"
Cc: Giovanni Campagna <gcampagna@src.gnome.org>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 431a1d12)