Documentation/drm: DocBook integration for DRRS

Adding an overview of DRRS in general and the implementation for eDP DRRS.
Also, describing the functions related to eDP DRRS.

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>