drm/i915/skl: Query DPLL attached to port on SKL

Modify the implementation to query DPLL attached to a SKL port.

v2: Rebase on top of the run-time PM on DPMS series (Damien)

v3: Modified as per review comments from Paulo

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 8432ae2..d67c59b 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -343,7 +343,10 @@
 	/* Selected dpll when shared or DPLL_ID_PRIVATE. */
 	enum intel_dpll_id shared_dpll;
 
-	/* PORT_CLK_SEL for DDI ports. */
+	/*
+	 * - PORT_CLK_SEL for DDI ports on HSW/BDW.
+	 * - enum skl_dpll on SKL
+	 */
 	uint32_t ddi_pll_sel;
 
 	/* Actual register state of the dpll, for shared dpll cross-checking. */