drm/amdgpu: optimize fn gfx_v11_ring_insert_nop

Optimize gfx_v11_ring_insert_nop() to call
optimized version of amdgpu_ring_insert_nop
instead of calling amdgpu_ring_write for number
of nop times.

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>