drm/panel/panel-sitronix-st7701: Split GIP and init sequences

The ST7701 initialization sequence is well parametrized, split the GIP
programming sequence, which is fully custom completely undocumented
TFT matrix specific magic register programming sequence into separate
callback so other TFT matrix definitions can add their own GIP sequence.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Guido Günther <agx@sigxcpu.org>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220710194437.289042-9-marex@denx.de