drm/amdgpu: use ARRAY_SIZE() to add amdgpu debugfs files

to easily add new debugfs file w/o changing the hardcoded list count.

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>