drm/msm/adreno: Allow SoC specific gpu device table entries

There are cases where there are differences due to SoC integration.
Such as cache-coherency support, and (in the next patch) e-fuse to
speedbin mappings.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/549767/