drm/amdgpu/display: fix warning when compiling without debugfs

fixes unused variable warning.

Reported-by: Eric Biggers <ebiggers@kernel.org>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>