msm: mdss: fix to return error when overlay get/set fail

Current calls to OVERLAY_SET and OVERLAY_GET ioctl are
not returning an error if the ioctl call fails since
the error gets overwritten by another function call.
Correcting this issue by preventing the return value
to get overwritten in case of failure.

Change-Id: I5d54ffdcdb86aa5d6521547bd40638dc4deb03da
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>