vga: optimize computation of dirty memory region

vga: optimize computation of dirty memory region

The depth == 0 and depth == 15 have to be special cased because
width * depth / 8 does not provide the correct scanline length.
However, thanks to the recent reorganization of vga_draw_graphic()
the correct value of VRAM bits per pixel is available in "bits".

Use it (via the same "bwidth" computation that is used later in
the function), thus restricting the slow path to the wraparound case.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>