drm/i915: use vma->node directly and rewrap map&fence in bind

Use () to make for neater alignment of the split lines, too. With this
we ditch another jump through the obj_gtt_size/offset indirection
maze.

Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>