drm/msm/adreno: Remove dead code

This BUG_ON will never be reached, and there is a comment 20 above
explaining why.

Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Reviewed-by: Akhil P Oommen <quic_akhilpo@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/487586/
Link: https://lore.kernel.org/r/20220528160353.157870-1-konrad.dybcio@somainline.org
Signed-off-by: Rob Clark <robdclark@chromium.org>