drm/i915: Add documentation to gen9_set_dc_state()

Add documentation to gen9_set_dc_state() on what enabling a given DC
state means and at what point HW/DMC actually enters/exits these states.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180417113147.25120-1-imre.deak@intel.com