cma: Make the default CMA region not reserved by default

Due to a bug in code logic, the default CMA region is
reserved when CONFIG_CMA_RESERVE_DEFAULT_AREA is NOT set
instead of when it is set. Fix this logic.

Change-Id: I476594c1e3745386741f2aba1b978a436b60c7a4
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>