drm/i915/psr: Comment to clarify SRD_DEBUG is called PSR_MASK SKL+

What was called SRD_DEBUG(0x6F860) on HSW and BDW was renamed to PSR_MASK
SKL onwards, add a note next to the macro definition.
There is also a different PSR_DEBUG on SKL+ to add to the confusion.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180313040954.6289-1-dhinakaran.pandiyan@intel.com