drm/amdgpu: add amdgpu_gart_map function v2

This allows us to write the mapped PTEs into
an IB instead of the table directly.

v2: fix build with debugfs enabled, remove unused assignment

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