drm/amdgpu: Limit vm max ctx number to 4096

driver need to reserve resource for each ctx for
some hw features. so add this limitation.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -164,6 +164,7 @@ extern int amdgpu_si_support;
 extern int amdgpu_cik_support;
 #endif
 
+#define AMDGPU_VM_MAX_NUM_CTX			4096
 #define AMDGPU_SG_THRESHOLD			(256*1024*1024)
 #define AMDGPU_DEFAULT_GTT_SIZE_MB		3072ULL /* 3GB by default */
 #define AMDGPU_WAIT_IDLE_TIMEOUT_IN_MS	        3000

drivers/gpu/drm/amd/amdgpu/amdgpu_ctx.c

@@ -248,7 +248,7 @@ static int amdgpu_ctx_alloc(struct amdgpu_device *adev,
 		return -ENOMEM;
 
 	mutex_lock(&mgr->lock);
-	r = idr_alloc(&mgr->ctx_handles, ctx, 1, 0, GFP_KERNEL);
+	r = idr_alloc(&mgr->ctx_handles, ctx, 1, AMDGPU_VM_MAX_NUM_CTX, GFP_KERNEL);
 	if (r < 0) {
 		mutex_unlock(&mgr->lock);
 		kfree(ctx);