drm/amdgpu: restructure amdgpu_fill_buffer v2

We ran into the problem that clearing really larger buffer (60GiB) caused an
SDMA timeout.

Restructure the function to use the dst window instead of mapping the whole
buffer into the GART and then fill only 2MiB/256MiB chunks at a time.

v2: rebase on restructured window map.

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