nvidia: add RmForceExternalGpu registry key

Thunderbolt 4/5 eGPU enclosures fail automatic external GPU detection
because they are not in the approved TB3 bridge list and do not report
the required slot capabilities (HotPlug+ and Surprise+).

Add RmForceExternalGpu registry key to bypass bridge detection and
unconditionally set PDB_PROP_GPU_IS_EXTERNAL_GPU, enabling proper eGPU
handling throughout the driver.

Usage:
  Global (all GPUs):
    NVreg_RegistryDwords="RmForceExternalGpu=1"

  Per-device (specific GPU only):
    NVreg_RegistryDwordsPerDevice="pci=DDDD:BB:DD.F;RmForceExternalGpu=1"

The per-device option allows targeting specific GPUs in multi-GPU systems.

Author: roger-pmta

src/nvidia/arch/nvalloc/unix/src/osinit.c

@@ -1310,10 +1310,29 @@ RmInitNvDevice(
     }
 
     // Configure eGPU setting
-    if (RmCheckForExternalGpu(pGpu, pCl))
     {
-        pGpu->setProperty(pGpu, PDB_PROP_GPU_IS_EXTERNAL_GPU, NV_TRUE);
-        nv->is_external_gpu = NV_TRUE;
+        NvU32 forceExternalGpu = 0;
+        NvBool isExternalGpu = NV_FALSE;
+
+        // Check if user has forced eGPU mode via RmForceExternalGpu registry key
+        if ((osReadRegistryDword(pGpu, NV_REG_STR_RM_FORCE_EXTERNAL_GPU, &forceExternalGpu) == NV_OK) &&
+            (forceExternalGpu != 0))
+        {
+            NV_PRINTF(LEVEL_NOTICE, "Forcing external GPU mode via RmForceExternalGpu registry key\n");
+            isExternalGpu = NV_TRUE;
+        }
+        else
+        {
+            // Otherwise, use automatic detection
+            isExternalGpu = RmCheckForExternalGpu(pGpu, pCl);
+        }
+
+        if (isExternalGpu)
+        {
+            NV_PRINTF(LEVEL_NOTICE, "GPU treated as external GPU (eGPU)\n");
+            pGpu->setProperty(pGpu, PDB_PROP_GPU_IS_EXTERNAL_GPU, NV_TRUE);
+            nv->is_external_gpu = NV_TRUE;
+        }
     }
     status->rmStatus = gpumgrStateInitGpu(pGpu);
     if (status->rmStatus != NV_OK)

src/nvidia/interface/nvrm_registry.h

@@ -1001,6 +1001,17 @@
 // If set, this will cause RM mark GPU as lost when it detects 0xFF from register
 // access.
 
+//
+// Type DWORD
+// Force external GPU mode, bypassing automatic Thunderbolt bridge detection.
+// Useful for Thunderbolt 4/5 enclosures not in the driver's approved bridge list.
+//
+// Encoding:
+//   0 (default) - use automatic eGPU detection
+//   1           - force GPU to be treated as external
+//
+#define NV_REG_STR_RM_FORCE_EXTERNAL_GPU                    "RmForceExternalGpu"
+
 #define NV_REG_STR_RM_BLACKLIST_ADDRESSES                "RmBlackListAddresses"
 // Type BINARY:
 // struct