drm/i915/hdmi: add debugfs to contorl HDMI bpc

drm/i915/hdmi: add debugfs to contorl HDMI bpc

While performing HDMI compliance testing, test equipment may request
different bpc output for signal measurement. However, display driver
typically determines the maximum available bpc based on HW bandwidth.
This change leverages the existing debugfs (intel_force_link_bpp)
to manage HDMI bpc, and making it easier to pass HDMI CTS.

v2: Using exist variable max_requested_bpc.
v3: Extend intel_force_link_bpp to support HDMI as suggested by Imre.
v4: Update commit message suggested by Jani.
v5: Remove unused header file.

Cc: Shankar Uma <uma.shankar@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Vidya Srinivas <vidya.srinivas@intel.com>
Signed-off-by: Lee Shawn C <shawn.c.lee@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250901055721.219995-2-shawn.c.lee@intel.com