drm/qxl: drop WARN_ONCE()

There is no good reason to flood the kernel log with a WARN
stacktrace just because someone tried to mmap a prime buffer.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190524104251.22761-1-kraxel@redhat.com