drm/amdgpu: use simpler remove_conflicting_pci_framebuffers()

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
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/fe59ab587ef4ef799b525fcc03d87800c5dc26d9.1535810304.git.mirq-linux@rere.qmqm.pl