drm/amdgpu/acpi: NULL check before some freeing functions is not needed

kfree(NULL) is safe, so removes NULL check before freeing the mem.
This patch also fix the ifnullfree.cocci warnings.
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
CC: Alex Deucher <alexander.deucher@amd.com>
CC: christian.koenig@amd.com
CC: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
CC: David Airlie <airlied@linux.ie> (maintainer:DRM DRIVERS)
CC: Lyude Paul <lyude@redhat.com>
CC: Rex Zhu <Rex.Zhu@amd.com>
CC: Jim Qu <Jim.Qu@amd.com>
CC: amd-gfx@lists.freedesktop.org
CC: dri-devel@lists.freedesktop.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -886,6 +886,5 @@ void amdgpu_acpi_get_backlight_caps(struct amdgpu_device *adev,
 void amdgpu_acpi_fini(struct amdgpu_device *adev)
 {
 	unregister_acpi_notifier(&adev->acpi_nb);
-	if (adev->atif)
-		kfree(adev->atif);
+	kfree(adev->atif);
 }