drm/msm/dsi: Simplify the logic in msm_dsi_manager_panel_init()

This patch moves things around a bit to be a little more readable and
pulls out the set_encoder_mode() call into its own function for later
use.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617201301.133275-7-sean@poorly.run