drm/msm/disp/mdp5: mdp5_cfg: Fix msm8974v2 max_clk

The maximum mdp clock rate on msm8974v2 is 320MHz. Fix it.

Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>