drm/i915/dpll: Accept intel_display as argument for shared_dpll_init

drm/i915/dpll: Accept intel_display as argument for shared_dpll_init

Use intel_display as an argument for intel_shared_dpll_init() and
replace drm_i915_private in function wherever possible.
While at it prefer using display->platform.xx over IS_PLATFORM.
Initialize dpio_phy and dpio_channel since with IS_GEMINILAKE()
and IS_BROXTON() compiler knows it will return false for xe but
since display->platform.xx is a runtime check which means the
compiler sees a potential path where uninitialized variables
could be accessed and raises a warning.

--v2
-Amend commit message to explain why some variables were
initialized [Jani]

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250212074542.3569452-8-suraj.kandpal@intel.com