drm/msm/dpu: initialise clk_rate to 0 in _dpu_core_perf_get_core_clk_rate

When removing the core perf tune overrides, I also occasionaly removed the
initialisation of the clk_rate variable. Initialise it to 0 to let max()
correctly calculate the maximum of requested clock rates.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Fixes: 6a4bc739 ("drm/msm/dpu: drop separate dpu_core_perf_tune overrides")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/551321/
Link: https://lore.kernel.org/r/20230804094804.36053-1-dmitry.baryshkov@linaro.org