drm_hwcomposer: Add MediaTek platform support

drm_hwcomposer: Add MediaTek platform support

This platform handler is dedicated for the i500 MediaTek SoC [1].
i500 has a Mali-G72 MP3 GPU.

OpenGL/Mali integration is based on ARM Gralloc module, version
BX304L01B-SW-99005-r20p0-01rel0, without additional patches.

This platformmediatek is based on platformmeson, without the additional
usage flag in the private_handle_t.
AFBC support has also been removed as it's unsupported.

External Android.bp file should be created in order to build this
module:

```
cc_library_shared {
    name: "hwcomposer.drm_mediatek",
    defaults: ["hwcomposer.drm_defaults"],
    srcs: [":drm_hwcomposer_platformmediatek"],
    whole_static_libs: ["drm_hwcomposer"],
}
```

[1] https://www.mediatek.com/products/AIoT/i500
Change-Id: I3ea7a980d76ba5c5ff583b5d4f21e1989875bafb
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>