drm/amdgpu: avoid integer overflow warning in amdgpu_device_resize_fb_bar()

On 32-bit architectures comparing a resource against a value larger than
U32_MAX can cause a warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:1344:18: error: result of comparison of constant 4294967296 with expression of type 'resource_size_t' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
                    res->start > 0x100000000ull)
                    ~~~~~~~~~~ ^ ~~~~~~~~~~~~~~

As gcc does not warn about this in dead code, add an IS_ENABLED() check at
the start of the function. This will always return success but not actually resize
the BAR on 32-bit architectures without high memory, which is exactly what
we want here, as the driver can fall back to bank switching the VRAM
access.

Fixes: 31b8adab ("drm/amdgpu: require a root bus window above 4GB for BAR resize")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -1325,6 +1325,9 @@ int amdgpu_device_resize_fb_bar(struct amdgpu_device *adev)
 	u16 cmd;
 	int r;
 
+	if (!IS_ENABLED(CONFIG_PHYS_ADDR_T_64BIT))
+		return 0;
+
 	/* Bypass for VF */
 	if (amdgpu_sriov_vf(adev))
 		return 0;