vgabios: Add support for reading framebuffer in "direct" mode

Support reading high memory "direct" framebuffers and translating the
results to 8 bit attribute values.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
vgabios: Add support handling text mode attributes while in graphics mode

Add support for simple text mode attribute emulation while in graphics
mode.  This seems to help with text highlighting and colors with a few
boot-loaders.  Turn it on only for CBVGA vgabios.

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