drm/mcde: Fix stability issue

Whenever a display update was sent, apart from updating
the memory base address, we called mcde_display_send_one_frame()
which also sent a command to the display requesting the TE IRQ
and enabling the FIFO.

When continuous updates are running this is wrong: we need
to only send this to start the flow to the display on
the very first update. This lead to the display pipeline
locking up and crashing.

Check if the flow is already running and in that case
do not call mcde_display_send_one_frame().

This fixes crashes on the Samsung GT-S7710 (Skomer).

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Stephan Gerhold <stephan@gerhold.net>
Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200718233323.3407670-1-linus.walleij@linaro.org