drm/radeon/kms: Fix NULL ptr dereference

radeon_atombios_fini might be call while there is not valid
atombios structure allocated, thus test for a not null ptr
before trying to access this structure.
Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/gpu/drm/radeon/radeon_device.c

@@ -487,8 +487,10 @@ int radeon_atombios_init(struct radeon_device *rdev)
 
 void radeon_atombios_fini(struct radeon_device *rdev)
 {
-	kfree(rdev->mode_info.atom_context->scratch);
-	kfree(rdev->mode_info.atom_context);
+	if (rdev->mode_info.atom_context) {
+		kfree(rdev->mode_info.atom_context->scratch);
+		kfree(rdev->mode_info.atom_context);
+	}
 	kfree(rdev->mode_info.atom_card_info);
 }