stdvga: report 6-bit DAC width

The stdvga hardware doesn't have a mechanism to change the DAC width,
but we can still implement the VBE Set/Get DAC Palette Format function
as specified: allow attempts to set the DAC width to anything greater
than or equal to 6 (which reports success but is otherwise a no-op), and
always return 6 when the DAC width is queried.

Signed-off-by: Daniel Verkamp <daniel@drv.nu>
Message-ID: <20260409065319.3438719-2-daniel@drv.nu>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>