drm/mediatek: Disable AFBC support on Mediatek DRM driver

drm/mediatek: Disable AFBC support on Mediatek DRM driver

Commit c410fa9b07c3 ("drm/mediatek: Add AFBC support to Mediatek DRM
driver") added AFBC support to Mediatek DRM and enabled the
32x8/split/sparse modifier.

However, this is currently broken on Mediatek MT8188 (Genio 700 EVK
platform); tested using upstream Kernel and Mesa (v25.2.1), AFBC is used by
default since Mesa v25.0.

Kernel trace reports vblank timeouts constantly, and the render is garbled:

```
[CRTC:62:crtc-0] vblank wait timed out
WARNING: CPU: 7 PID: 70 at drivers/gpu/drm/drm_atomic_helper.c:1835 drm_atomic_helper_wait_for_vblanks.part.0+0x24c/0x27c
[...]
Hardware name: MediaTek Genio-700 EVK (DT)
Workqueue: events_unbound commit_work
pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : drm_atomic_helper_wait_for_vblanks.part.0+0x24c/0x27c
lr : drm_atomic_helper_wait_for_vblanks.part.0+0x24c/0x27c
sp : ffff80008337bca0
x29: ffff80008337bcd0 x28: 0000000000000061 x27: 0000000000000000
x26: 0000000000000001 x25: 0000000000000000 x24: ffff0000c9dcc000
x23: 0000000000000001 x22: 0000000000000000 x21: ffff0000c66f2f80
x20: ffff0000c0d7d880 x19: 0000000000000000 x18: 000000000000000a
x17: 000000040044ffff x16: 005000f2b5503510 x15: 0000000000000000
x14: 0000000000000000 x13: 74756f2064656d69 x12: 742074696177206b
x11: 0000000000000058 x10: 0000000000000018 x9 : ffff800082396a70
x8 : 0000000000057fa8 x7 : 0000000000000cce x6 : ffff8000823eea70
x5 : ffff0001fef5f408 x4 : ffff80017ccee000 x3 : ffff0000c12cb480
x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff0000c12cb480
Call trace:
 drm_atomic_helper_wait_for_vblanks.part.0+0x24c/0x27c (P)
 drm_atomic_helper_commit_tail_rpm+0x64/0x80
 commit_tail+0xa4/0x1a4
 commit_work+0x14/0x20
 process_one_work+0x150/0x290
 worker_thread+0x2d0/0x3ec
 kthread+0x12c/0x210
 ret_from_fork+0x10/0x20
---[ end trace 0000000000000000 ]---
```

Until this gets fixed upstream, disable AFBC support on this platform, as
it's currently broken with upstream Mesa.

Fixes: c410fa9b07c3 ("drm/mediatek: Add AFBC support to Mediatek DRM driver")
Cc: stable@vger.kernel.org
Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Macpaul Lin <macpaul.lin@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20251024202756.811425-1-ariel.dalessandro@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>