mdss: rotator: move buffer mapping in workqueue from caller context

Rotator driver maps the same buffer twice. First time in caller
context while second time in worker queue. Ideally, it should
map buffer single time in worker queue. Mapping it in caller
context during validation will block the client call and can
cause the delay. Moving it to worker thread fixes double
mapping issue for same buffer.

Change-Id: I17ebe188ace2e54714247fc87277f3da51a64d73
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>