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.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>