dma-mapping: Always remap on dma buffer free

DMA clients may opt to clear cpu virtual address mapping
on alloc. On free when dma buffer is given back to cpu,
ensure that it is always remapped in cpu address space.

Change-Id: I67f9616d9bee8cf07cbe81468ebf174d2599f4c5
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>