drm/vc4: use simpler remove_conflicting_framebuffers(NULL)

Use remove_conflicting_framebuffers(NULL) instead of open-coding it.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Eric Anholt <eric@anholt.net>
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/be7e6396e6e7159695a5bccc109c6ab49a51e0d8.1535810304.git.mirq-linux@rere.qmqm.pl