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>

diff --cc Documentation/DocBook/drm.tmpl
Simple merge

diff --cc drivers/gpu/drm/i915/intel_psr.c
Simple merge