drm/msm/dsi: drop PLL accessor functions

Replace PLL accessor functions (pll_read/pll_write*) with the DSI PHY
accessors, reducing duplication.

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/20210331105735.3690009-20-dmitry.baryshkov@linaro.org


Signed-off-by: Rob Clark <robdclark@chromium.org>