drm/i915: Add PSR docbook

Let's document PSR a bit. No functional changes.

v2: Add actual DocBook entry and accept Daniel's improvements.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
2 files changed