tests: use drmtest_skip() in caching ioctl helpers

This way we can rip out all the skip handling from the test control flow,
and additionally (by using drmtest_retval()) even get correct exit codes.

The only tricky part is that when we only want ot skip parts of a test
(like for gem_pread and gem_pwrite) we need to split out those parts as
subtests. But no addition of control-flow is required, the set/longjmp
magic in the helpers all makes it happen.

Also we make extensive use of the behaviour of drmtest_skip to skip
all subsequent subtests if it is called outside of a subtest. This allows
us to re-flatten the control flow a lot.

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

tests/gem_partial_pwrite_pread.c

@@ -257,17 +257,8 @@ static void do_tests(int cache_level, const char *suffix)
 {
 	char name[80];
 
-	if (cache_level != -1) {
-		switch (gem_set_caching(fd, scratch_bo->handle, cache_level)) {
-		case 0: break;
-		case -EINVAL:
-		case -ENOTTY:
-			return;
-		default:
-			assert(0);
-			return;
-		}
-	}
+	if (cache_level != -1)
+		gem_set_caching(fd, scratch_bo->handle, cache_level);
 
 	snprintf(name, sizeof(name), "reads%s", suffix);
 	drmtest_subtest_block(name)
@@ -315,5 +306,5 @@ int main(int argc, char **argv)
 
 	close(fd);
 
-	return 0;
+	return drmtest_retval();
 }