drm/i915/slpc: Update the frequency debugfs

Read the values stored in the SLPC structures. Remove the
fields that are no longer valid (like RPS interrupts) as
well.

v2: Move all functionality changes to this patch (Jani)
v3: Fix compile warning and if condition (Jani)

Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221005155943.34747-3-vinay.belgaumkar@intel.com