drm/vc4: Fix a memory leak in hang state error path

In the Linux kernel, the following vulnerability has been resolved: drm/vc4: Fix a memory leak in hang state error path When vc4_save_hang_state() encounters an early return condition, it returns without freeing the previously allocated `kernel_state`, leaking memory. Add the missing kfree() calls by consolidating the early return paths into a single place.