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.