drm/i915/bdw: Add Broadwell support for debugfs rps freq info

Add Broadwell support to i915_frequency_info
and extend i915_max|min_freq_get|set to (gen >= 6).

v2: generalized support for i915_max|min_freq_get|set (Daniel).

Signed-off-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Reviewed-by: Jeff McGee <jeff.mcgee@intel.com>
[danvet: Fix checkpatch fail.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>