Yet another attempt to make palette loading more safer:

• Add a separate palette data for 8-bit DAC mode when SC_PIXEL_MODE is set

and fill it up with default gray-scale palette data for text. Now we don't
have to set `hint.sc.0.vesa_mode' to get the default palette data.

• Add a new adapter flag, V_ADP_DAC8 to track whether the controller is

using 8-bit palette format and load correct palette when switching modes.

• Set 8-bit DAC mode only for non-VGA compatible graphics mode.