drm/i915: rename intel_pm.[ch] to intel_clock_gating.[ch]

Observe that intel_pm.[ch] is now purely about clock gating, so rename
them to intel_clock_gating.[ch]. Rename the functions to
intel_clock_gating_*() to follow coding conventions.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230403122428.3526263-1-jani.nikula@intel.com