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>

Author: Perry Yuan

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

@@ -2597,18 +2597,22 @@ static int amdgpu_pmops_suspend(struct device *dev)
 	else if (amdgpu_acpi_is_s3_active(adev))
 		adev->in_s3 = true;
 	if (!adev->in_s0ix && !adev->in_s3) {
+#if IS_ENABLED(CONFIG_SUSPEND)
 		/* don't allow going deep first time followed by s2idle the next time */
 		if (adev->last_suspend_state != PM_SUSPEND_ON &&
 		    adev->last_suspend_state != pm_suspend_target_state) {
 			drm_err_once(drm_dev, "Unsupported suspend state %d\n",
 				     pm_suspend_target_state);
 			return -EINVAL;
 		}
+#endif
 		return 0;
 	}
 
+#if IS_ENABLED(CONFIG_SUSPEND)
 	/* cache the state last used for suspend */
 	adev->last_suspend_state = pm_suspend_target_state;
+#endif
 
 	return amdgpu_device_suspend(drm_dev, true);
 }