drm/amdgpu: skip reservation of discovery tmr region in pre-Navi

IP discovery is only supported in Navi series and onwards.
There is no need to reserve a portion of vram as discovery
tmr region for pre-Navi adapters.
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1961,6 +1961,7 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
 	 * reserve TMR memory at the top of VRAM which holds
 	 * IP Discovery data and is protected by PSP.
 	 */
+	if (adev->discovery_tmr_size > 0) {
 		r = amdgpu_bo_create_kernel_at(adev,
 			adev->gmc.real_vram_size - adev->discovery_tmr_size,
 			adev->discovery_tmr_size,
@@ -1969,6 +1970,7 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
 			NULL);
 		if (r)
 			return r;
+	}
 
 	DRM_INFO("amdgpu: %uM of VRAM memory ready\n",
 		 (unsigned) (adev->gmc.real_vram_size / (1024 * 1024)));