drm/msm/dsi: Don't store dsi host mode_flags in msm_dsi

It's a bit dangerous to store the flags in msm_dsi since there's no way to
tell when they're populated. Fortunately the only place that uses them
is the same place that fills them. So just use a local variable and
delete the struct member.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617201301.133275-5-sean@poorly.run