drm/amdgpu: unify BO evicting method in amdgpu_ttm

Unify BO evicting functionality for possible memory
types in amdgpu_ttm.c.

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