drm/msm/dsi_pll_7nm: Fix variable usage for pll_lockdet_rate

The PLL_LOCKDET_RATE_1 was being programmed with a hardcoded value
directly, but the same value was also being specified in the
dsi_pll_regs struct pll_lockdet_rate variable: let's use it!

Based on 362cadf3

 ("drm/msm/dsi_pll_10nm: Fix variable usage for
pll_lockdet_rate")

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>