drm/ttm: remove TTM_MEMTYPE_FLAG_CMA

The original intention was to avoid CPU page table unmaps
when BOs move between the GTT and SYSTEM domain.

The problem is that this never correctly handled changes
in the caching attributes or backing pages.

Just drop this for now and simply unmap the CPU page
tables in all cases.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/378240/