drm/qxl: Remove dead code

The QXL driver sets DRIVER_MODESET unconditionally, so testing for the
absence of the feature will always fail.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466788520-21325-5-git-send-email-frank.binns@imgtec.com