msm: camera: Fix bug in dma_buf_vunmap

Unmapping was happening in reverse order. Due to this, dma vmap ptr
address validation was failing for cnt 1 while unmapping, and observing
the device crash. This change fix this issue.

Change-Id: I35d889235b7d8ce93d8e382b2683f4882bc77c8d
Signed-off-by: Abhishek Jain <abhij@codeaurora.org>