drm/i915/selftests: Run the perf MI_BB tests on gen4/5

Now that we know the ring timestamp frequency on gen4/5 we
can run the perf tests that depend on sampling the timestamp.

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

When executing the read via the CS i965 doesn't seem to need
the double read trick that CPU mmio reads need.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221031135703.14670-7-ville.syrjala@linux.intel.com
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>