drm/i915: Tidy aliasing_gtt_bind_vma()

drm/i915: Tidy aliasing_gtt_bind_vma()

In commit 4bb7569f5b62e9ada44ab06837178b0f5e71c0f9
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Oct 15 14:23:01 2015 +0200

    drm/i915: restore ggtt double-bind avoidance

we wrote the ggtt_bind_vma() observing a number of cleanups we could do
over the template of aliasing_gtt_bind_vma(). Now let's apply the
cleanups we made there back to the original. The essence is to avoid
redundant variables and assignements, and by doing so make the code
easier to read.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1448015238-24639-1-git-send-email-chris@chris-wilson.co.uk