drm/qxl: reapply cursor after resetting primary

QXL associates mouse state with its primary plane.

Destroying a primary plane and putting a new one in place has the side
effect of destroying the cursor as well.

This commit changes the driver to reapply the cursor any time a new
primary is created. It achieves this by keeping a reference to the
cursor bo on the qxl_crtc struct.

This fix is very similar to

commit 4532b241 ("drm/qxl: reapply cursor after SetCrtc calls")

which got implicitly reverted as part of implementing the atomic
modeset feature.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1512097


Fixes: 1277eed5 ("drm: qxl: Atomic phase 1: convert cursor to universal plane")
Cc: stable@vger.kernel.org
Signed-off-by: Ray Strode <rstrode@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>