drm/i915/selftests: Test RING_TIMESTAMP on gen4/5

Now that we actually know the cs timestamp frequency on gen4/5
let's run the corresponding test.

On g4x/ilk we must read the udw of the 64bit timestamp
register. Details in {g4x,gen5)_read_clock_frequency().

The one extra caveat is that on i965 (or at least CL, don't
recall if I ever tested on BW) we must read the register
twice to get an up to date value. For some unknown reason
the first read tends to return a stale value.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221031135703.14670-6-ville.syrjala@linux.intel.com
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>