drm/i915: Show actual alongside requested frequency in debugfs/i915_rps_boost_info

Previously we hesitated in adding the hw probe for the actual GPU
frequency for rps_boost as it is quite cumbersome, but given some
surprising HW behaviour it would be useful to know both the RPS boost
state and the actual HW state in one location.

v2: vlv/chv needs more tlc

Reported-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181002113221.29208-1-chris@chris-wilson.co.uk