drm/i915/debugfs: New debugfs for display clock frequencies

Instead of mixing display & non-display stuff together, move
display specific clock info to new debugfs. This patch will
create a new debugfs "i915_cdclk_info" to expose Current & Max
cdclk and Max pixel clock frequency info.

Example:
$ cat /sys/kernel/debug/dri/0/i915_cdclk_info
Current CD clock frequency: 163200 kHz
Max CD clock frequency: 652800 kHz
Max pixel clock frequency: 1305600 kHz

V2: - s/i915_display_clock_info/i915_cdclk_info/ (Jani)
    - Move the logic to intel_cdclk.c (Jani)
    - Don't remove info from i915_frequency_info (Jani)
V3: - Drop locking (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230413114502.1105288-1-bhanuprakash.modem@intel.com