drm/i915/pxp: prefer forward declaration over includes

Always use forward declarations instead of includes in headers if
possible.
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NMatt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214173644.2097124-1-jani.nikula@intel.com

drivers/gpu/drm/i915/pxp/intel_pxp_pm.h

@@ -6,7 +6,7 @@
 #ifndef __INTEL_PXP_PM_H__
 #define __INTEL_PXP_PM_H__
 
-#include "intel_pxp_types.h"
+struct intel_pxp;
 
 #ifdef CONFIG_DRM_I915_PXP
 void intel_pxp_suspend_prepare(struct intel_pxp *pxp);