randr: unref the provider shared pixmap the appropriate number of times

randr: unref the provider shared pixmap the appropriate number of times

When an RandR shared pixmap is created in rrCreateSharedPixmap, it has a refcnt
of 1.  Then, PixmapShareToSlave bumps the refcnt to 2.  However, there's no
corresponding PixmapUnshareFromSlave where the refcnt can be decreased again,
and there's no convenient common place where the refcnt can be decremented when
the slave pixmap is destroyed.

Fix this by just unreffing the pixmap twice in RRCrtcDetachScanoutPixmap.

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>