drm/msm/dsi: simplify vco_delay handling in dsi_phy_28nm driver

Instead of setting the variable and then using it just in the one place,
determine vco_delay directly at the PLL configuration time.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Tested-by: Stephen Boyd <swboyd@chromium.org> # on sc7180 lazor
Link: https://lore.kernel.org/r/20210331105735.3690009-16-dmitry.baryshkov@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>