msm: mdss: Add error handling after mapping buffer

Currently buffer is marked to mapped even though
ion_map_iommu() returned failure. This might end up
unmapping memory that was never mapped in the first
place. Fix this by adding proper error handling.

Change-Id: Ie29127af2bc39df607a232ee5e0646dfe3411dee
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>