drm/panel: st7701: Swap vertical front and back porch timings

Vertical front and back porch values on existing driver are swapped.
The existing timings are still working as expected, but to make sure
it can compatible with techstar ts8550b bsp timings this patch swap
the same values.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190512184827.13905-1-jagan@amarulasolutions.com