drm/amdgpu: cleanup debugfs for amdgpu rings

Use debugfs_create_file_size API for creating ring debugfs, and as its a
NULL returning API, change the return type for amdgpu_debugfs_ring_init
API as well. Also cleanup surrounding code.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>