drm/msm/a5xx: really check for A510 in a5xx_gpu_init

The commit 010c8bba ("drm: msm: adreno: Disable preemption on Adreno
510") added special handling for a510 (this SKU doesn't seem to support
preemption, so the driver should clamp nr_rings to 1). However the
gpu->revn is not yet set (it is set later, in adreno_gpu_init()) and
thus the condition is always false. Check config->rev instead.

Fixes: 010c8bba ("drm: msm: adreno: Disable preemption on Adreno 510")
Reported-by: Adam Skladowski <a39.skl@gmail.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Adam Skladowski <a39.skl@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/531511/


Signed-off-by: Rob Clark <robdclark@chromium.org>