drm/msm/dsi: set encoder mode for DRM bridge explicitly

Currently, DRM bridge for DPU relies on the default video
mode setting to set the encoder mode.

Add an explicit call to set the encoder mode for bridges.

Changes in v3:
- None

Reviewed-by: Archit Taneja <architt@codeauorora.org>
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>