drm/qxl: remove conflicting framebuffers earlier

Add error checking while being at it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190301092502.30948-4-kraxel@redhat.com