drm/msm/dsi: drop multiple pll enable_seq support

The only PLL using multiple enable sequences is the 28nm PLL, which just
does the single step in the loop. Push that support back into the PLL
code.

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/20210331105735.3690009-6-dmitry.baryshkov@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>