drm/i915: Simplify processing of the golden render context state

Rewrite i915_gem_render_state.c for the purposes of clarity and
compactness, in the process we can eliminate some dodgy math that did
not handle 64bit addresses correctly.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>