drm/amdgpu: remove unnecessary checks

amdgpu_ttm_backend_bind() only needed for TTM_PL_TT
and AMDGPU_PL_PREEMPT.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>