drm/ast: fix the ast open key function

When porting from UMS I mistyped this from the wrong place, AST noticed
and pointed it out, so we should fix it to be like the X.org driver.
Reported-by: NY.C. Chen <yc_chen@aspeedtech.com>
Cc: stable@vger.kernel.org
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/ast/ast_drv.h

@@ -177,7 +177,7 @@ uint8_t ast_get_index_reg_mask(struct ast_private *ast,
 
 static inline void ast_open_key(struct ast_private *ast)
 {
-	ast_set_index_reg_mask(ast, AST_IO_CRTC_PORT, 0xA1, 0xFF, 0x04);
+	ast_set_index_reg(ast, AST_IO_CRTC_PORT, 0x80, 0xA8);
 }
 
 #define AST_VIDMEM_SIZE_8M    0x00800000