gpu/drm/amdgpu: Fix build when CONFIG_DEBUG_FS is not set

If the CONFIG_DEBUG_FS is not selected, compilation of the
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c provides two warnings that
amdgpu_debugfs_regs_init and amdgpu_debugfs_regs_cleanup are used but
never defined. And as result:

ERROR: "amdgpu_debugfs_regs_cleanup" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
ERROR: "amdgpu_debugfs_regs_init" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
            ^
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>