drm/i915/selftests: Show the pcode frequency table on error

If we encounter an error while scaling, read back the frequency tables
from the pcu.

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/20200420172739.11620-5-chris@chris-wilson.co.uk