drm/i915/rps: Add frequency translation helpers

Add two helpers that for reading the actual GT's frequency. The
two helpers are:

 - intel_rps_read_cagf: reads the frequency and returns it not
   normalized

 - intel_rps_read_actual_frequency: provides the frequency in Hz.

Use the above helpers in sysfs and debugfs.

Signed-off-by: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213183736.31992-2-andi@etezian.org