vgabios: Refactor vga_set_mode and stdvga_set_mode.

vgabios: Refactor vga_set_mode and stdvga_set_mode.

Split out the BDA setup part of vga_set_mode to new function
modeswitch_set_bda.  Move the remaining parts (palette loading, screen
clearing, font loading) of vga_set_mode into stdvga_set_mode.

Add new mode switching flags and pass them to stdvga_set_mode, so it
does not need to inspect modeset_ctl directly.

Move code needed by stdvga_set_mode (perform_gray_scale_summing,
clear_screen) to stdvga.c.

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