Digital gain registers are comprised of 13 bits. 4 bits [12:9]
control the integer portion, and 9 bits [8:0] control the decimal
portion. Therefore 256 (0x100) corresponds to ~0.5x digital gain.

This sets digital gain to 1.0x by default and limits the minimum
value to 1.0x since there's little point in having global digital
gain of less than 1.0x as its only effect is lowering picture quality
by reducing its dynamic range and white point.