drm/i915: Extract i915_cs_timestamp_{ns_to_ticks,tick_to_ns}()

Pull the code to do the CS timestamp ns<->ticks conversion into
helpers and use them all over.

The check in i915_perf_noa_delay_set() seems a bit dubious,
so we switch it to do what I assume it wanted to do all along
(ie. make sure the resulting delay in CS timestamp ticks
doesn't exceed 32bits)?

Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200302143943.32676-5-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>