drm/msm: Fix bv_fence being used as bv_rptr

The bv_fence field of rbmemptrs was being used incorrectly as the BV
rptr shadow pointer in some places.

Add a bv_rptr field and change the code to use that instead.

Reviewed-by: Akhil P Oommen <quic_akhilpo@quicinc.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8650-QRD
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-QRD
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8450-HDK
Signed-off-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/618010/


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