drm/gma500: Fix potential NULL pointer dereference

fb is being dereferenced before it is null checked, hence there
is a potential null pointer dereference.

Fix this by moving the pointer dereference after fb has been
properly null checked at line 74: if (!fb)

Addresses-Coverity-ID: 1470169 ("Dereference before null check")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180625121844.GA12466@embeddedor.com

drivers/gpu/drm/gma500/gma_display.c

@@ -60,7 +60,7 @@ int gma_pipe_set_base(struct drm_crtc *crtc, int x, int y,
 	struct drm_psb_private *dev_priv = dev->dev_private;
 	struct gma_crtc *gma_crtc = to_gma_crtc(crtc);
 	struct drm_framebuffer *fb = crtc->primary->fb;
-	struct gtt_range *gtt = to_gtt_range(fb->obj[0]);
+	struct gtt_range *gtt;
 	int pipe = gma_crtc->pipe;
 	const struct psb_offset *map = &dev_priv->regmap[pipe];
 	unsigned long start, offset;
@@ -76,6 +76,8 @@ int gma_pipe_set_base(struct drm_crtc *crtc, int x, int y,
 		goto gma_pipe_cleaner;
 	}
 
+	gtt = to_gtt_range(fb->obj[0]);
+
 	/* We are displaying this buffer, make sure it is actually loaded
 	   into the GTT */
 	ret = psb_gtt_pin(gtt);