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.