drm/mediatek: mtk_hdmi_v2: Add debugfs ops and implement ABIST

drm/mediatek: mtk_hdmi_v2: Add debugfs ops and implement ABIST

Implement the Automated Built-In Self-Test ABIST functionality
provided by the HDMIv2 IP and expose it through the "hdmi_abist"
debugfs file.

Write "1" to this file to activate ABIST, or "0" to deactivate.

The ABIST functionality can be used to validate that the HDMI
Transmitter itself works and that can output a valid image to
the HDMI Display that is connected.

This is especially useful when trying to rule out any possible
issue that is related to the display pipeline, as the HDMI Tx
is always the last component; this means that HDMI ABIST can be
used even without prior display controller pipeline configuration.

The expected output is a 100% color bar (rainbow) test pattern.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Louis-Alexis Eyraud <louisalexis.eyraud@collabora.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20251023-mediatek-drm-hdmi-v2-v11-10-7873ec4a1edf@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>