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/kms_flip.c

@@ -628,8 +628,6 @@ static void hang_gpu(struct test_output *o)
 	int fd;
 	ssize_t r;
 
-	igt_assert(card_index != -1);
-
 	snprintf(fname, FILENAME_MAX, "%s/%i/%s",
 		 dfs_base, card_index, dfs_entry);