drm/i915/dpll: Move away from using shared dpll

Rename functions to move away from using shared dpll in the dpll
framework as much as possible since dpll may not always be shared.

--v2
-Use intel_dpll_global instead of global_dpll [Jani]

--v3
-Just use intel_dpll [Jani]

--v4
-Drop the global from comments [Jani]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20250515071801.2221120-8-suraj.kandpal@intel.com