drm/i915/display: remove unnecessary i915_debugfs.h includes

Leftovers from before display debugfs was separated to its own file.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230411131922.401602-1-jani.nikula@intel.com