gbm: coding style cleanup
gbm: coding style cleanup

Waffle aims to keep the main code path in it's core, with the
error path diverging via goto labels and alike.

Follow the approach and update the gbm code.

Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Reviewed-by: Chad Versace <4b94d436ff8c93527a96d251ac64b06311506efe@chromium.org>

chadversary/waffle,waffle-gl/waffle,dcbaker/waffle,chadversary/waffle,waffle-gl/waffle,chadversary/waffle,dcbaker/waffle,dcbaker/waffle,waffle-gl/waffle,waffle-gl/waffle