drm/i915/display: debugfs entry to list display capabilities

Create a separate debugfs entry to list the display capabilities
IGT can rely on this debugfs entry for tests that depend on
display device and display runtime info for both xe and i915
drivers.

v2: rename the entry to i915_display_capabilities (Chaitanya)

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231018102723.16915-2-vinod.govindapillai@intel.com