lib/drmtest: igt_skip when drm_get_card fails

Also add printf support to igt_skip to make sure there's always a
reason why we fail a testcase.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

tests/gem_seqno_wrap.c

@@ -644,7 +644,6 @@ int main(int argc, char **argv)
 	parse_options(argc, argv);
 
 	card_index = drm_get_card();
-	igt_assert(card_index != -1);
 
 	srandom(time(NULL));