cma: Increase number of retries for allocation

It was observed that CMA pages are sometimes getting
pinned down by BG process scheduled out in their exit
path. Since BG process has lower priority they end up
getting less time slice by scheduler there by consuming
more time to free up CMA page.

So instead of failing to allocate and directly
returning error on CMA allocation path, we increase the
number of retries to 2 to see if the process which
was in exit path and about to release pages  was able to
to do so or not.

Change-Id: I693228d36186ab17480f36b492ada91ab7c262d8
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>