drm/mediatek: mtk_dpi: Move pixel clock setting flow to function

drm/mediatek: mtk_dpi: Move pixel clock setting flow to function

In preparation for adding support for the DPI IP found in MT8195
and in MT8188 used for HDMI, move the code flow for calculation
and setting of the DPI pixel clock to a separate function called
mtk_dpi_set_pixel_clk().

This was done because, on those platforms, the DPI instance that
is used for HDMI will get its pixel clock from the HDMI clock,
hence it is not necessary, nor desirable, to calculate or set
the pixel clock in DPI.

Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20250217154836.108895-5-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>