dma-contiguous: Re-order the error handling sequence

When CMA allocation fails because of any pending
signals, we clear off pfn and bit map before
returning back. While clearing off the bitmap,
we still use the pfn info so, clear pfn only
after bitmap is cleared.

CRs-fixed: 772299
Change-Id: I94e566181f75b7c8ebdab7d29437e5fca5f3fbdc
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>