drm/i915/dpll: Rename crtc_get_shared_dpll

Rename crtc_get_shared_dpll to take into the individual PLL framework
which came in at DISPLAY_VER >= 14.
Also having shared dpll stuff also in intel_dpll.c is just confusing.

--v2
-Change naming to dpll_global to keep consistency with rest of the
naming

--v3
-Just use intel_dpll [Jani]

--v4
-Modify commit message [Jani]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20250515071801.2221120-9-suraj.kandpal@intel.com