fbdev: allow apertures == NULL in remove_conflicting_framebuffers()

Interpret (otherwise-invalid) NULL apertures argument to mean all-memory
range. This will allow to remove several duplicates of this code
from drivers in following patches.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/b5fed539fe569035139c8594c23effd8c39f12ca.1535810304.git.mirq-linux@rere.qmqm.pl