glamor: Invalidate cached GEM handle in radeon_set_pixmap_bo

We continued using the stale cached handle, causing issues e.g. when
resizing the screen via RandR.

Reported-by: iive on IRC
Acked-by: Alex Deucher <alexander.deucher@amd.com>

src/radeon.h

@@ -735,6 +735,7 @@ static inline Bool radeon_set_pixmap_bo(PixmapPtr pPix, struct radeon_buffer *bo
 		    return TRUE;
 
 		radeon_buffer_unref(&priv->bo);
+		priv->handle_valid = FALSE;
 	    }
 
 	    drmmode_fb_reference(pRADEONEnt->fd, &priv->fb, NULL);