drm/panel/panel-sitronix-st7701: Adjust porch control bitfield name

Define DSI_CMD2_BK0_PORCTRL_VBP_MASK and DSI_CMD2_BK0_PORCTRL_VFP_MASK
and move the vertical back and front porch calculation from macros into
the st7701_init_sequence() function, so it is clear what this does.

No functional change.

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/20220710194437.289042-6-marex@denx.de