drm/amdgpu: clean unused functions of uvd/vcn/vce

Some of the functions pointers of amdgpu_ip_funcs
are not used and are left commented out. Hence this
cleans those up which arent used.

Cc: Leo Liu <leo.liu@amd.com>
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>