drm/panel/panel-sitronix-st7701: Clean up CMDnBKx selection

There are two command register files, CMD1 and CMD2, where only the CMD2
contains additional register sub-files BK0..3 . Pull the register file
selection call into separate function instead of duplicating it all over
the driver. The CMD2BK2 file is undocumented in datasheet, and is used
for BIST. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014231042.468033-1-marex@denx.de