drm/i915: Drop return value from intel_fill_fb_ggtt_view

It can't fail and there's even a WARN_ON suggesting that if it would,
it would be a disaster.

Correct this to make things less confusing.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1444834266-12689-1-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>