drm/i915/pxp: fix undefined reference to `intel_pxp_gsccs_is_ready_for_sessions'

On x86_64 with gcc version 13.3.0, I compile kernel with:

  make defconfig
  ./scripts/kconfig/merge_config.sh .config <(
    echo CONFIG_COMPILE_TEST=y
  )
  make KCFLAGS="-fno-inline-functions -fno-inline-small-functions -fno-inline-functions-called-once"

Then I get a linker error:

  ld: vmlinux.o: in function `pxp_fw_dependencies_completed':
  kintel_pxp.c:(.text+0x95728f): undefined reference to `intel_pxp_gsccs_is_ready_for_sessions'

This is caused by not having a intel_pxp_gsccs_is_ready_for_sessions()
header stub for CONFIG_DRM_I915_PXP=n. Add it.

Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
Fixes: 99afb7c ("drm/i915/pxp: Add ARB session creation and cleanup")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20250415090616.2649889-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
(cherry picked from commit b484c1e)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Author: black-desk

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

@@ -23,6 +23,7 @@ int intel_pxp_gsccs_init(struct intel_pxp *pxp);
 
 int intel_pxp_gsccs_create_session(struct intel_pxp *pxp, int arb_session_id);
 void intel_pxp_gsccs_end_arb_fw_session(struct intel_pxp *pxp, u32 arb_session_id);
+bool intel_pxp_gsccs_is_ready_for_sessions(struct intel_pxp *pxp);
 
 #else
 static inline void intel_pxp_gsccs_fini(struct intel_pxp *pxp)
@@ -34,8 +35,11 @@ static inline int intel_pxp_gsccs_init(struct intel_pxp *pxp)
 	return 0;
 }
 
-#endif
+static inline bool intel_pxp_gsccs_is_ready_for_sessions(struct intel_pxp *pxp)
+{
+	return false;
+}
 
-bool intel_pxp_gsccs_is_ready_for_sessions(struct intel_pxp *pxp);
+#endif
 
 #endif /*__INTEL_PXP_GSCCS_H__ */