Use corect depth from DisplaySurface in vmware_vga.c

Hello,
for what I can tell, there is no way for vmware_vga to work correctly
right now. It assumes that the framebuffer bits-per-pixel and the one
from the DisplaySurface are identical (it uses directly the VRAM from
vga.c), but it always assumes 3 bytes per pixel, which is never possible
with the current version of DisplaySurface.
Attached patch fixes that by using ds_get_bits_per_pixel.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>