drm/i915/selftests: Unroll the CS frequency loop

Having noticed that MI_BB_START is incurring a memory stall (see the
correlation with uncore frequency), we have to unroll the loop in order
to diminish the impact of the MI_BB_START on the instruction throughput.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200421171351.19575-1-chris@chris-wilson.co.uk