drm/i915: Store CS timestamp frequency in Hz

kHz isn't accurate enough for storing the CS timestamp
frequency on some of the platforms. Store the value
in Hz instead.

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/20200302143943.32676-2-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>