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>
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_fb.c b/drivers/gpu/drm/vmwgfx/vmwgfx_fb.c
index 191f1b2..070797b 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_fb.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_fb.c
@@ -613,8 +613,7 @@
 	par->bo_ptr = NULL;
 	ttm_bo_kunmap(&par->map);
 
-	/* move this to system instead of just unpinning it */
-	vmw_dmabuf_to_system(vmw_priv, par->vmw_bo, false);
+	vmw_dmabuf_unpin(vmw_priv, par->vmw_bo, false);
 
 	return 0;
 }