drm/qxl: stop using TTM_MEMTYPE_FLAG_MAPPABLE v2

The driver doesn't expose any not-mapable memory resources.

v2: remove unused man variable as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/378246/