drm/ttm: nuke memory type flags

It's not supported to specify more than one of those flags.
So it never made sense to make this a flag in the first place.

Nuke the flags and specify directly which memory type to use.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/389826/?series=81551&rev=1