drm/msm/dsi_pll_10nm: 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 10nm:
remove the TODO by simply using that variable.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>