drm/msm/dp: move/inline panel related functions

Move panel-related functions to dp_panel.c, following up the cleanup
done by the rest of the submodules.

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/654321/
Link: https://lore.kernel.org/r/20250518-fd-dp-audio-fixup-v6-6-2f0ec3ec000d@oss.qualcomm.com