drm/msm/adreno: Add support for Adreno 510 GPU

drm/msm/adreno: Add support for Adreno 510 GPU

The Adreno 510 GPU is a stripped version of the Adreno 5xx,
found in low-end SoCs like 8x56 and 8x76, which has 256K of
GMEM, with no GPMU nor ZAP.
Also, since the Adreno 5xx part of this driver seems to be
developed with high-end Adreno GPUs in mind, and since this
is a lower end one, add a comment making clear which GPUs
which support is not implemented yet is not using the GPMU
related hw init code, so that future developers will not go
crazy with that.

By the way, the lower end Adreno GPUs with no GPMU are:
A505/A506/A510 (usually no ZAP firmware)
A508/A509/A512 (usually with ZAP firmware)

Signed-off-by: AngeloGioacchino Del Regno <kholk11@gmail.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>