drm/i915: Update i915_gem_gtt.c copyright

I keep meaning to do this... by now almost the entire file has been
written by an Intel employee (including Daniel post-2010).

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>