drm/ttm: remove TTM_MEMTYPE_FLAG_MAPPABLE

Not used any more. And it is bad design to use a TTM flag
to do a check inside a driver.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/378245/