drm/mcde: Rename flow function

The function mcde_display_send_one_frame() has a historical
name that stems from being implemented when the driver
only supported single frame updates.

Rename it mcde_start_flow() so that it reflects the current
usage.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200729090915.252730-1-linus.walleij@linaro.org