drm/msm/a6xx: Move a6xx_bus_clear_pending_transactions to a6xx_gpu

drm/msm/a6xx: Move a6xx_bus_clear_pending_transactions to a6xx_gpu

This function is responsible for telling the GPU to halt transactions
on all of its relevant buses, drain them and leave them in a predictable
state, so that the GPU can be e.g. reset cleanly.

Move the function to a6xx_gpu.c, remove the static keyword and add a
prototype in a6xx_gpu.h to accomodate for the move.

Reviewed-by: Akhil P Oommen <quic_akhilpo@quicinc.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/542762/
Signed-off-by: Rob Clark <robdclark@chromium.org>