drm/msm/a6xx: Correct the highestbank configuration

Highest bank bit configuration is different for a618 gpu. Update
it with the correct configuration which is the reset value incidentally.

Signed-off-by: Akhil P Oommen <akhilpo@codeaurora.org>
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>
Fixes: e812744c ("drm: msm: a6xx: Add support for A618")
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>