drm/vc4: allocate enough memory in vc4_save_hang_state()

"state" is smaller than "kernel_state" so we end up corrupting memory.

Fixes: 21461365 ('drm/vc4: Add an interface for capturing the GPU state after a hang.')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Eric Anholt <eric@anholt.net>