msm: mdss: Copy only error code back to userspace after atomic commit

Previously, the atomic commit ioctl copies the entire kernel layer list
to the userspace layer list structure. The only parameter modified
during atomic commit should be the error code, and therefore, the
only value needed by userspace after the atomic commit ioctl returns.
Copying the kernel layer list structure causes the userspace to have
stale kernel pointer references to scale and PP info structures.

Change-Id: Ia8e96af21f7d9594a47d1503f3afef50a767971f
Signed-off-by: Benet Clark <benetc@codeaurora.org>