msm: display: check get_unused_fd_flags return val

Not checking the error code, which might be negative, will cause us to wrongly
reference fdt[fd] by using the error code, causing kernel panic in fd_install.

Change-Id: I1f8e68e74bdbdd5b81bc3450f7d93b5cd96c9439
Signed-off-by: Iliyan Malchev <malchev@google.com>
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>