vgabios: Fix stdvga_perform_gray_scale_summing().

Fix conversion error that resulted in
stdvga_perform_gray_scale_summing not actually writing the new results
back.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
vgabios: Fix cirrus memory clear on mode switch.

The cirrus_clear_vram() code wasn't actually doing anything because of
a u8 overflow.  Fix that.

Fill with 0xff when performing a legacy cirrus mode switch (WinXP has
been observed to incorrectly render dialog boxes if the memory is
filled to 0).  This was the behavior of the original LGPL vgabios
code.  To support this, add mechanism (MF_LEGACY) to allow vga drivers
to detect if the mode switch is from vesa or int10.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>