drm/amdgpu: stop using TTM_MEMTYPE_FLAG_MAPPABLE

The driver does support some not-mapable resources, but
those are already handled correctly in the switch/case
statement in the code.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/378243/