drm/i915/selftests: Measure CS_TIMESTAMP

Count the number of CS_TIMESTAMP ticks and check that it matches our
expectations.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200521071059.31726-1-chris@chris-wilson.co.uk