[media] s5p-mfc: Fix input/output format reporting

The video encode/decode paths have duplicated logic between
VIDIOC_TRY_FMT and VIDIOC_S_FMT that should be de-duped.  Also, video
decode reports V4L2_PIX_FMT_NV12MT_16X16 output format, regardless of
what the actual output has been set at.  Fix this.

Signed-off-by: John Sheu <sheu@google.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>