msm: mdss: Fix Null Pointer Dereference in unmap error case

If buffer mapping is unsuccessful we are unmapping buffer and
returning. In such error case after returning, we are again
unmapping the buffer second time due to which NULL pointer
dereference occurs. Fix this by removing the second unmap.

Change-Id: Ic3a133a423b80b762c1d408824d984063b013f31
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>