drm/i915/cx0: Add MTL+ .enable_clock/.disable clock hooks on DDI

drm/i915/cx0: Add MTL+ .enable_clock/.disable clock hooks on DDI

To enable pll clock on DDI move part of the pll enabling
sequence into a ddi clock enabling function.

Simililarly, do the same for pll disabling sequence.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/20251117104602.2363671-30-mika.kahola@intel.com