drm/qxl: switch qxl to &drm_gem_object_funcs.mmap

Wire up the new drm_gem_ttm_mmap() helper function.
Use generic drm_gem_mmap() and remove qxl_mmap().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20191017132638.9693-3-kraxel@redhat.com