drm/msm/dpu: add missing flush and fetch bits for DMA4/DMA5 planes

Note that with this, DMA4/DMA5 are still non-functional, but at least
display *something* in modetest instead of nothing or underflow.

Fixes: efcd0107 ("drm/msm/dpu: add support for SM8550")
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Tested-by: Neil Armstrong <neil.armstrong@linaro.org> # on SM8550-QRD
Patchwork: https://patchwork.freedesktop.org/patch/545548/
Link: https://lore.kernel.org/r/20230704160106.26055-1-jonathan@marek.ca


Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>