drm/msm/dp: split MMSS_DP_DSC_DTO register write to a separate function

It's the dp_panel's duty to clear the MMSS_DP_DSC_DTO register. Once DP
driver gets DSC support, it will handle that register in other places
too. Split a call to write 0x0 to that register to a separate function.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Tested-by: Stephen Boyd <swboyd@chromium.org> # sc7180-trogdor
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/654314/
Link: https://lore.kernel.org/r/20250518-fd-dp-audio-fixup-v6-1-2f0ec3ec000d@oss.qualcomm.com