msm: vidc: Get rid of `map_output_buffer` in struct msm_vidc_inst

map_output_buffer was added to distinguish between kernel and userspace
clients in order to skip mapping buffers into IOMMU.  We no longer
support kernel clients, so get rid of stale code.

Change-Id: Ifb1d1b4fb38e46013513dfbe856812ecb5ca98c3
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>