drm/msm/dpu: Add handling of LM_6 and LM_7 bits in pending flush mask

MDSS/MDP v12 comes with new bits in flush registers (e.g.
MDP_CTL_0_FLUSH) for Layer Mixer 6 and 7.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/651260/
Link: https://lore.kernel.org/r/20250430-b4-sm8750-display-v5-13-8cab30c3e4df@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>