mesa/gbm: Resolve GCC sign-compare warning.

mesa/gbm: Resolve GCC sign-compare warning.

mesa/src/gbm/main/backend.c: In function 'find_backend':
mesa/src/gbm/main/backend.c:70:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < ARRAY_SIZE(backends); ++i) {
                  ^
mesa/src/gbm/main/backend.c: In function '_gbm_create_device':
mesa/src/gbm/main/backend.c:95:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (i = 0; i < ARRAY_SIZE(backends) && dev == NULL; ++i) {
                  ^

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>