drm/amdgpu: Fix clang warning: unused label 'exit'

Problem:
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:332:1: warning: unused label 'exit' [-Wunused-label]
exit:
^~~~~

Fix: Put #ifdef CONFIG_64BIT around exit

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210525184431.1170373-1-andrey.grodzovsky@amd.com