drm/ast: fixed cursor may disappear sometimes

BugLink: https://bugs.launchpad.net/bugs/1810967

commit 7989b9ee upstream.
Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/gpu/drm/ast/ast_mode.c

@@ -1250,7 +1250,7 @@ static int ast_cursor_move(struct drm_crtc *crtc,
 	ast_set_index_reg(ast, AST_IO_CRTC_PORT, 0xc7, ((y >> 8) & 0x07));
 
 	/* dummy write to fire HWC */
-	ast_set_index_reg_mask(ast, AST_IO_CRTC_PORT, 0xCB, 0xFF, 0x00);
+	ast_show_cursor(crtc);
 
 	return 0;
 }