drm/i915: Fix cs timestamp frequency for cl/bw

Despite what the spec says the TIMESTAMP register seems to
tick once every hrawclk (confirmed on i965gm and g35).

v2: Rebase

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221031135703.14670-4-ville.syrjala@linux.intel.com


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