lib/core: Add igt_reset_timeout

Convenience wrapper suggested by Chris for igt_set_timeout(0, NULL).

v2: While at it add an empty line in kms_flip to make
set/reset_timeout a visual block.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

tests/kms_flip.c

@@ -1616,7 +1616,8 @@ static void test_nonblocking_read(int in)
 
 	igt_set_timeout(5, "Nonblocking DRM fd reading");
 	ret = read(fd, buffer, sizeof(buffer));
-	igt_set_timeout(0, NULL);
+	igt_reset_timeout();
+
 	igt_assert_eq(ret, -1);
 	igt_assert_eq(errno, EAGAIN);