drm/i915: Clear the stolen fb before enabling

As the stolen memory region will contain the contents of whatever was
last there, it invariably contains garbage. To be consistent with the
shmemfs backed fb and the expectations of the fb layer, we need to clear
the fb prior to installing it as an fbcon.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58111


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Fixup sparse __iomem confusion reported by Wu Fengguang.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>