xf86drm: Fix type-punned pointer build warning

      CC       libdrm_la-xf86drmMode.lo
    ../xf86drmMode.c: In function 'drmHandleEvent':
    ../xf86drmMode.c:854:15: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
       e = (struct drm_event *)(&buffer[i]);
                   ^

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99350


Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>