xf86drm: fix sign-compare warning

xf86drm.c:3601:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     while (expected < sizeof(match)) {
                     ^

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>