vmwgfx: minor dmabuf utilities cleanup

Add / fix some function comments.
Don't move out an fbdev framebuffer when unused. Just unpin.
Only have a single function that computes a SVGAGuestPtr from the buffer's
current placement, and make it more versatile by accepting a
struct ttm_buffer_object

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>