drm/cirrus: use simpler remove_conflicting_pci_framebuffers()

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/a9a4691b6eea789664edd6b85a85b7990953a222.1535810304.git.mirq-linux@rere.qmqm.pl