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

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.

Fixes: 214613656b51 ("drm/vc4: Add an interface for capturing the GPU state after a hang.")
Reviewed-by: Melissa Wen <mwen@igalia.com>
Link: https://patch.msgid.link/20260330-vc4-misc-fixes-v1-3-92defc940a29@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>