drm/msm/dsi_pll_10nm: Fix dividing the same numbers twice

In function dsi_pll_calc_dec_frac we are calculating the decimal
div start parameter by dividing the decimal multiple by the
fractional multiplier: the remainder of that operation is stored
to then get programmed to the fractional divider registers of
the PLL.

It's useless to call div_u64_rem to get the remainder and *then*
call div_u64 to get the division result, as the first is already
giving that result: let's fix it by just caring about the result
of div_u64_rem.

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