drm/i915: Stop claiming cs timestamp frquency on gen2/3

Gen2/3 have no TIMESTAMP registers to sample so no point in thinking
we have any frequency for it either.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221031135703.14670-3-ville.syrjala@linux.intel.com


Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>