drm/panel/panel-sitronix-st7701: Make gamma correction TFT specific

The gamma correction values are specific to the TFT which is attached to
the ST7701 TFT matrix driver, move the gamma correction values from what
incorrectly looks like common init sequence into TFT matrix specific
settings.

While doing so, add macros which defined fields within the gamma register
file and a macro which mimics FIELD_PREP except works with constant
expressions.

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/20220710194437.289042-4-marex@denx.de