disp: msm: sde: check correct field for XIN idle status

Current code is checking the HALT_ACK instead of HALT_IDLE
fields which leads to unnecessarily issuing halt requests
on ports that are already idle.

Change-Id: I2269ce6364dab3bb8a52a0fb1b1e68e9b92b6444
Signed-off-by: Steve Cohen <cohens@codeaurora.org>