drm/i915/cdclk: switch to new platform checks

Switch the IS_<PLATFORM>() checks to display->platform.<platform>, and
drop a number of struct drm_i915_private pointers in the process.

While at it, replace

	/* NOOP */;

with
	; /* NOOP */

to avoid a checkpatch warning on misleading indentation.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250204134228.2934744-2-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>