drm/i915/display: identify display steppings in display probe

Both i915 and xe have code to identify display steppings. Start
deduplicating this by, uh, adding a third copy in display code. This is
not yet used for anything other than debug logging. We'll switch over
later.

For platforms before GMD ID, attach the mapping from PCI revision to
stepping in the platform and subplatform descriptors. This is a
considerably cleaner approach than having it completely separate.

Also add a separate field for stepping in display runtime info,
preserving the value from GMD ID.

v2: Handle NULL subdesc (Matt)
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240821095036.2044654-1-jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_display_device.h

@@ -201,8 +201,9 @@ struct intel_display_runtime_info {
 	struct intel_display_ip_ver {
 		u16 ver;
 		u16 rel;
-		u16 step;
+		u16 step; /* hardware */
 	} ip;
+	int step; /* symbolic */
 
 	u32 rawclk_freq;
 

drivers/gpu/drm/xe/compat-i915-headers/intel_step.h

@@ -8,6 +8,7 @@
 
 #include "xe_step.h"
 
+#define intel_step xe_step
 #define intel_step_name xe_step_name
 
 #endif /* __INTEL_STEP_H__ */