drm/amdgpu: Fix build error when CONFIG_SUSPEND is disabled

The variable `pm_suspend_target_state` is conditionally defined only when
`CONFIG_SUSPEND` is enabled (see `include/linux/suspend.h`). Directly
referencing it without guarding by `#ifdef CONFIG_SUSPEND` causes build
failures when suspend functionality is disabled (e.g., `CONFIG_SUSPEND=n`).

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>