[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.