drm/i915: Propagate errors back from fb set-base

Along the modesetting short cut where we skip trying to do a full
modeset and instead simply update the framebuffer base registers, we
failed to handle any errors reported.

This regression has been introduced in

commit 94352cf9
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jul 5 22:51:56 2012 +0200

    drm/i915: push crtc->fb update into pipe_set_base

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>