msm: vidc: Set buffer types properly in prepare_buf for encoder

Due to how HAL buffer types are tied into the secure hints, switch up
the HAL buffer types if the current session is of type encoder.

Change-Id: Idb74340ffad25278dd926c869745e235b4821ae3
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>