drm/msm/dp: break up dp_display_enable into two parts

drm/msm/dp: break up dp_display_enable into two parts

dp_display_enable() currently re-trains the link if needed and then
enables the pixel clock, programs the controller to start sending the
pixel stream. Split these two parts into prepare/enable APIs, to support
MST bridges_enable insert the MST payloads funcs between enable
stream_clks and program register.

Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Signed-off-by: Yongxing Mou <yongxing.mou@oss.qualcomm.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/742740/
Link: https://lore.kernel.org/r/20260728-dp_mstclean-v9-8-f7779fce10f4@oss.qualcomm.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>