drm/mcde: Support using DSI in LP mode

It is possible to set a flag in the struct mipi_dsi_device
so the panel is handled in low power (LP) mode. Some displays
only support this mode and it is also good for testing.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: newbytee@protonmail.com
Cc: Stephan Gerhold <stephan@gerhold.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200808223122.1492124-2-linus.walleij@linaro.org