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

drivers/gpu/drm/i915/i915_reg.h

@@ -4180,13 +4180,13 @@ enum {
 #define EDP_PSR_PERF_CNT		_MMIO(dev_priv->psr_mmio_base + 0x44)
 #define   EDP_PSR_PERF_CNT_MASK		0xffffff
 
-#define EDP_PSR_DEBUG				_MMIO(dev_priv->psr_mmio_base + 0x60)
+#define EDP_PSR_DEBUG				_MMIO(dev_priv->psr_mmio_base + 0x60) /* PSR_MASK on SKL+ */
 #define   EDP_PSR_DEBUG_MASK_MAX_SLEEP         (1<<28)
 #define   EDP_PSR_DEBUG_MASK_LPSP              (1<<27)
 #define   EDP_PSR_DEBUG_MASK_MEMUP             (1<<26)
 #define   EDP_PSR_DEBUG_MASK_HPD               (1<<25)
 #define   EDP_PSR_DEBUG_MASK_DISP_REG_WRITE    (1<<16)
-#define   EDP_PSR_DEBUG_EXIT_ON_PIXEL_UNDERRUN (1<<15)
+#define   EDP_PSR_DEBUG_EXIT_ON_PIXEL_UNDERRUN (1<<15) /* SKL+ */
 
 #define EDP_PSR2_CTL			_MMIO(0x6f900)
 #define   EDP_PSR2_ENABLE		(1<<31)