msm_fb: Avoid MDP mixer reset from ISR context

During DSI error interrupt handling, we currently try to do
MDP mixer reset. Avoid this since MDP clock control happens
in mixer reset which is not correct in ISR context. Schedule
a workqueue instead to call mixer reset as a separate thread.

CRs-fixed: 508882
Change-Id: Ie5471a01d79b4d8b67d8bb78647c407c35acae65
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>