msm: mdss: unmap rotator src buffer after mapping the next buffer

In secure case, when same buffer is queued in multiple
composition cycles we should map it before unmapping it
so that TZ will not clear the buffer content.

In the former case ref count can be zero which can lead to TZ
clearing the buffer

Change-Id: I5aea01f8d2c4330b93319434ced55f816558619d
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>