drm/i915: Add kerneldoc comments to the intel_context struct

A bit of background on the context elements.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Appease checkpatch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>