drm/i915: Specify when the PLL hw state fields are valid

Not all those fields are valid on a given platform. Make it explicit.

Unions could also be used, but were cluttering some code paths with
if/else ladders.

v2: Don't use anonymous unions (Daniel)
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -197,10 +197,13 @@ enum intel_dpll_id {
 #define I915_NUM_PLLS 2
 
 struct intel_dpll_hw_state {
+	/* i9xx, pch plls */
 	uint32_t dpll;
 	uint32_t dpll_md;
 	uint32_t fp0;
 	uint32_t fp1;
+
+	/* hsw, bdw */
 	uint32_t wrpll;
 };