drm/msm/dsi: Split mode_flags out of msm_dsi_host_get_panel()

We use the flags in more places than just get_panel, so split them out
into a separate function.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617201301.133275-4-sean@poorly.run