drm/vmwgfx: Set dmabuf_size when vmw_dmabuf_init is successful

SOU primary plane prepare_fb hook depends upon dmabuf_size to pin up BO
(and not call a new vmw_dmabuf_init) when a new fb size is same as
current fb. This was changed in a recent commit which is causing
page_flip to fail on VM with low display memory and multi-mon failure
when cycle monitors from secondary display.

Cc: <stable@vger.kernel.org> # 4.14, 4.16
Fixes: 20fb5a635a ("drm/vmwgfx: Unpin the screen object backup buffer when not used")
Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_scrn.c

@@ -482,6 +482,8 @@ vmw_sou_primary_plane_prepare_fb(struct drm_plane *plane,
 		return ret;
 	}
 
+	vps->dmabuf_size = size;
+
 	/*
 	 * TTM already thinks the buffer is pinned, but make sure the
 	 * pin_count is upped.