drm/msm/dsi: remove msm_dsi_pll_set_usecase

msm_dsi_pll_set_usecase() function is not used outside of individual DSI
PHY drivers, so drop it in favour of calling the the respective
set_usecase functions directly.

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/20210331105735.3690009-10-dmitry.baryshkov@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>