drm/amdgpu: Fix size validation for non-exclusive domains (v4)

Fix amdgpu_bo_validate_size() to check whether the TTM domain manager for the
requested memory exists, else we get a kernel oops when dereferencing "man".

v2: Make the patch standalone, i.e. not dependent on local patches.
v3: Preserve old behaviour and just check that the manager pointer is not
    NULL.
v4: Complain if GTT domain requested and it is uninitialized--most likely a
    bug.

Cc: Alex Deucher <Alexander.Deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: AMD Graphics <amd-gfx@lists.freedesktop.org>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -448,27 +448,24 @@ static bool amdgpu_bo_validate_size(struct amdgpu_device *adev,
 
 	/*
 	 * If GTT is part of requested domains the check must succeed to
-	 * allow fall back to GTT
+	 * allow fall back to GTT.
 	 */
 	if (domain & AMDGPU_GEM_DOMAIN_GTT) {
 		man = ttm_manager_type(&adev->mman.bdev, TTM_PL_TT);
 
-		if (size < man->size)
+		if (man && size < man->size)
 			return true;
-		else
-			goto fail;
-	}
-
-	if (domain & AMDGPU_GEM_DOMAIN_VRAM) {
+		else if (!man)
+			WARN_ON_ONCE("GTT domain requested but GTT mem manager uninitialized");
+		goto fail;
+	} else if (domain & AMDGPU_GEM_DOMAIN_VRAM) {
 		man = ttm_manager_type(&adev->mman.bdev, TTM_PL_VRAM);
 
-		if (size < man->size)
+		if (man && size < man->size)
 			return true;
-		else
-			goto fail;
+		goto fail;
 	}
 
-
 	/* TODO add more domains checks, such as AMDGPU_GEM_DOMAIN_CPU */
 	return true;