drm/radeon/kms: add bounds checking to avivo pll algo

Prevent divider overflow.
Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=28932

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_display.c

@@ -793,6 +793,11 @@ static void avivo_get_fb_div(struct radeon_pll *pll,
 	tmp *= target_clock;
 	*fb_div = tmp / pll->reference_freq;
 	*frac_fb_div = tmp % pll->reference_freq;
+
+        if (*fb_div > pll->max_feedback_div)
+		*fb_div = pll->max_feedback_div;
+        else if (*fb_div < pll->min_feedback_div)
+                *fb_div = pll->min_feedback_div;
 }
 
 static u32 avivo_get_post_div(struct radeon_pll *pll,
@@ -826,6 +831,11 @@ static u32 avivo_get_post_div(struct radeon_pll *pll,
 			post_div--;
 	}
 
+	if (post_div > pll->max_post_div)
+		post_div = pll->max_post_div;
+	else if (post_div < pll->min_post_div)
+		post_div = pll->min_post_div;
+
 	return post_div;
 }