drm/i915: deduplicate frequency dump on debugfs

Although commit 9dd4b065 ("drm/i915/gt: Move pm debug files into a
gt aware debugfs") says it was moving debug files to gt/, the
i915_frequency_info file was left behind and its implementation copied
into drivers/gpu/drm/i915/gt/debugfs_gt_pm.c. Over time we had several
patches having to change both places to keep them in sync (and some
patches failing to do so). The initial idea was to remove
i915_frequency_info, but there are user space tools using it. From a
quick code search there are other scripts and test tools besides igt, so
it's not simply updating igt to get rid of the older file.

Here we export a function using drm_printer as parameter and make
both show() implementations to call this same function. Aside from a few
variable name differences, for i915_frequency_info this brings a few
lines that were not previously printed: RP UP EI, RP UP THRESHOLD, RP
DOWN THRESHOLD and RP DOWN EI.  These came in as part of
commit 9c878557

 ("drm/i915/gt: Use the RPM config register to
determine clk frequencies"), which didn't change both places.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210918025754.1254705-4-lucas.demarchi@intel.com