drm/ttm/amdgpu: consolidate ttm reserve paths

Drop the WARN_ON and consolidate the two paths into one.

Use the consolidate slowpath in the execbuf utils code.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804025632.3868079-6-airlied@gmail.com