drm/i915/perf: reuse timestamp frequency from device info

Now that we have this stored in the device info, we can drop it from perf
part of the driver.

Note that this requires to init perf after we've computed the frequency,
hence why we move i915_perf_init() from i915_driver_init_early() to after
intel_device_info_runtime_init().

v2: Use div_u64 (Chris)

v3: Drop u64 divs by switching to kHz (Chris/Ville)
    Move i915_perf_fini to i915_driver_cleanup_hw (Matthew)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171113181902.12411-2-lionel.g.landwerlin@intel.com