fbdev: arkfb: Remove the unused function dac_read_reg()

The function dac_read_reg() is defined in the arkfb.c file, but not
called elsewhere, so delete this unused function.

drivers/video/fbdev/arkfb.c:322:18: warning: unused function 'dac_read_reg'.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2155


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Helge Deller <deller@gmx.de>