drm/msm/dpu: Stop copying around mode->private_flags

The driver never sets mode->private_flags so copying
it back and forth is entirely pointless. Stop doing it.

Also drop private_flags from the tracepoint.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: linux-arm-msm@vger.kernel.org
Cc: freedreno@lists.freedesktop.org
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428171940.19552-5-ville.syrjala@linux.intel.com