[media] s5p-mfc: Move firmware allocation point to avoid allocation problems

Move firmware allocation from open to probe to avoid problems
when using CMA for allocation. In certain circumstances CMA may allocate
buffer that is not in the beginning of the MFC memory area.

Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>