drm/i915: Use old mbus_join value when increasing CDCLK

In order to make sure we are not breaking the proper sequence
let's do updates step by step and don't change MBUS join value
during MDCLK/CDCLK programming stage.
MBUS join programming would be taken care by pre/post ddb hooks.

v2: - Reworded comment about using old mbus_join value in
      intel_set_cdclk(Ville Syrjälä)

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
[v3: vsyrjala: rebase on top of cdclk changes, reword a bit more]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240402155016.13733-11-ville.syrjala@linux.intel.com