drm/i915/debugfs: remove i915_display_capabilities

Turns out we've added two similar debugfs files. Consolidate on
intel_display_caps as it has more info and a driver independent name.

IGT has already switched over to intel_display_caps in IGT commit
cf837fc17d6c ("lib/dsc: use intel_display_caps instead of
i915_display_capabilities").

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://lore.kernel.org/r/241c5886cf2e95c694a693bb1b1953f6ae15390e.1744295009.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>