drm/msm/a690: Switch to a660_gmu.bin

There isn't actually a a690_gmu.bin.  But it appears that the normal
a660_gmu.bin works fine.  Normally all the devices within a sub-
generation (or "family") will use the same fw, and a690 is in the a660
family.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Fixes: 5e7665b5 ("drm/msm/adreno: Add Adreno A690 support")
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/552406/