drm/gma500: fix potential NULL pointer dereference dereference

NULL check at line 528: if (!sender || !data_out || !len_out) {, implies
that pointer _sender_ might be NULL.

Move pointer _sender_ dereference after NULL check in order to avoid a
potential NULL pointer dereference.

This issue was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170812015515.GA8360@embeddedgus

drivers/gpu/drm/gma500/mdfld_dsi_pkg_sender.c

@@ -520,7 +520,7 @@ static int __read_panel_data(struct mdfld_dsi_pkg_sender *sender, u8 data_type,
 			u8 *data, u16 len, u32 *data_out, u16 len_out, bool hs)
 {
 	unsigned long flags;
-	struct drm_device *dev = sender->dev;
+	struct drm_device *dev;
 	int i;
 	u32 gen_data_reg;
 	int retry = MDFLD_DSI_READ_MAX_COUNT;
@@ -530,6 +530,8 @@ static int __read_panel_data(struct mdfld_dsi_pkg_sender *sender, u8 data_type,
 		return -EINVAL;
 	}
 
+	dev = sender->dev;
+
 	/**
 	 * do reading.
 	 * 0) send out generic read request