drivers: dma: map pages only if required

DMA clients such as PIL would often allocate large
contiguous physical memory through dma-removed interface
for carve-outs. It may not be possible to map these large
allocations due to insufficient vmalloc space.

Hence provide provision to map the allocated pages only
if mapping or zeroing is explicitly requested for.

Change-Id: I1bfea1223e9ba9819b7768e01d5910690ea0f281
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>