drm/omapdrm: use BUG_ON macro for error debugging.

Since the if statement only checks for the value of the `id` variable,
it can be replaced by the more concise BUG_ON() macro for error
reporting.
Issue found using coccinelle.
Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200102095515.7106-1-wambui.karugax@gmail.com

drivers/gpu/drm/omapdrm/dss/dispc.c

@@ -393,8 +393,7 @@ static void dispc_get_reg_field(struct dispc_device *dispc,
 				enum dispc_feat_reg_field id,
 				u8 *start, u8 *end)
 {
-	if (id >= dispc->feat->num_reg_fields)
-		BUG();
+	BUG_ON(id >= dispc->feat->num_reg_fields);
 
 	*start = dispc->feat->reg_fields[id].start;
 	*end = dispc->feat->reg_fields[id].end;