drm/msm/dsi_pll_10nm: 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!

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>