[media] exynos4-is: Free FIMC-IS CPU memory only when allocated

Ensure dma_free_coherent() is not called with incorrect arguments
and only when the memory was actually allocated. This will prevent
possible crashes on error paths of the top level media device driver,
when fimc-is device gets unregistered and its driver detached.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: stable@vger.kernel.org # for v3.15
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>