drm/i915: add GPU max frequency control file

Mainly for use in debugging and benchmarking, this file allows the user
to control the max frequency used by the GPU.  Frequency may still vary
based on workload (if the frequency is set to higher than the minimum)
but won't go over the newly set value.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>