drm/i915/pm: use forward declaration to remove an include

The fewer includes the better.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dcb426e4c6497f9ee3356c5f4fd4aaabd6295262.1629281426.git.jani.nikula@intel.com

drivers/gpu/drm/i915/intel_pm.h

@@ -8,7 +8,6 @@
 
 #include <linux/types.h>
 
-#include "display/intel_bw.h"
 #include "display/intel_display.h"
 #include "display/intel_global_state.h"
 
@@ -19,6 +18,7 @@ struct drm_device;
 struct drm_i915_private;
 struct i915_request;
 struct intel_atomic_state;
+struct intel_bw_state;
 struct intel_crtc;
 struct intel_crtc_state;
 struct intel_plane;