drm/i915: add kerneldoc for intel_lr_context_size()

This function was recently renamed & exposed, so now it gets documented

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1451996493-16079-1-git-send-email-david.s.gordon@intel.com