drm/i915: Make skl_compute_dbuf_slices() behave consistently for all platforms

Currently skl_compute_dbuf_slices() returns 0 for any inactive pipe on
icl+, but returns BIT(S1) on pre-icl for any pipe (whether it's active or
not). Let's make the behaviour consistent and always return 0 for any
inactive pipe.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225171125.28885-6-ville.syrjala@linux.intel.comReviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -4606,7 +4606,7 @@ static u8 skl_compute_dbuf_slices(const struct intel_crtc_state *crtc_state,
 	 * For anything else just return one slice yet.
 	 * Should be extended for other platforms.
 	 */
-	return BIT(DBUF_S1);
+	return active_pipes & BIT(pipe) ? BIT(DBUF_S1) : 0;
 }
 
 static u64