drm/msm/dsi_pll_7nm: Solve TODO for multiplier frac_bits assignment

The number of fractional registers bits is known and already set in
the frac_bits variable of the dsi_pll_config struct here in 7nm:
remove the TODO by simply using that variable. This is a copy of
196145eb

 ("drm/msm/dsi_pll_10nm: Solve TODO for multiplier frac_bits
assignment").

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>