drm/i915: split out i915_timer_util.[ch]

Move timer related utilities from i915_utils.[ch] to separate new files
i915_timer_util.[ch]. Clean up related includes.

Note: Arguably none of this should exist in i915 in the first place. At
least isolate it better.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/0a83d9489626121dcefcd4c1a05317399b5708f3.1757582214.git.jani.nikula@intel.com


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