drm/vc4: copy_to_user() returns the number of bytes remaining

The copy_to/from_user() functions return the number of bytes remaining
to be copied.  We want to return error codes here.

Also it's a bad idea to print an error message if a copy from user fails
because users can use that to spam /var/log/messages which is annoying
so I removed those.

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>