ANDROID: GKI: Add DRM_TTM config to GKI

When certain DRM drivers, such as virtio-gpu, are built out of tree,
they might depend on parts of DRM, such as DRM_TTM, that are not
included in all configs.

This CL adds the config to GKI_HACKS_TO_FIX, allowing DRM drivers that
use DRM_TTM to link successfully against GKI.

Test: Build and run virtio-gpu as an external module
Change-Id: I1ae869b7394a6059c0f3880bdc65a0b3ceb3e004
Bug: 153580313
Signed-off-by: Lingfeng Yang <lfy@google.com>