vgabios: Change vga_set_mode() to directly setup BDA.

vgabios: Change vga_set_mode() to directly setup BDA.

The calls to set_cursor_shape, set_cursor_pos, and set_active_page
have no impact other than to set the BDA variables, because the
standard vga mode switch already programed the registers these
functions set.  So, just setup the BDA directly.

The stdvga_set_text_block_specifier call is unnecessary - it sets
a register which is already programmed that way.

Call stdvga_get_crtc() to obtain the crtc address instead of inferring
it from the memory model.

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