CMA: correct unlock target

'cma: Remove potential deadlock situation' introduces per cma area mutex
for bitmap management. It is good, but there is one mistake. When we
can't find appropriate area in bitmap, we release cma_mutex global lock
rather than cma->lock and this is a bug. So fix it.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>