drm/gma500: fix null dereference of pointer fb before null check

Pointer fb is being dereferenced when assigning dev before it
is null checked.  Fix this by only dereferencing dev after the
null check.

Fixes: 6b7ce2c4 ("drm/gma500: Remove struct psb_fbdev")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191216162136.270114-1-colin.king@canonical.com

drivers/gpu/drm/gma500/accel_2d.c

@@ -228,8 +228,8 @@ static void psbfb_copyarea_accel(struct fb_info *info,
 {
 	struct drm_fb_helper *fb_helper = info->par;
 	struct drm_framebuffer *fb = fb_helper->fb;
-	struct drm_device *dev = fb->dev;
-	struct drm_psb_private *dev_priv = dev->dev_private;
+	struct drm_device *dev;
+	struct drm_psb_private *dev_priv;
 	uint32_t offset;
 	uint32_t stride;
 	uint32_t src_format;
@@ -238,6 +238,8 @@ static void psbfb_copyarea_accel(struct fb_info *info,
 	if (!fb)
 		return;
 
+	dev = fb->dev;
+	dev_priv = dev->dev_private;
 	offset = to_gtt_range(fb->obj[0])->offset;
 	stride = fb->pitches[0];