drm/mcde: dsi: Enable clocks in pre_enable() instead of mode_set()

The DSI initialization sequence incorrectly assumes that the mode_set()
function of the DRM bridge is always called when (re-)enabling the display.
This is not necessarily the case.

Keeping the device idle in the framebuffer console for a while results
in the display being turned off using the disable() function. However,
as soon as any key is pressed only (pre_)enable() are called.
mode_set() is skipped because the mode has not been changed.

In this case, the DSI HS/LP clocks are never turned back on,
preventing the display from working.

Fix this by moving a part of the initialization sequence from
mode_set() to pre_enable(). Keep most of the video mode setup in
mode_set() since most of the registers are only dependent on the mode
that is set for the panel - there is no need to write them again each
time we re-enable the display.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106165835.2863-7-stephan@gerhold.net