drm/rockchip: fix NULL check on devm_kzalloc() return value

The right variable to check here is port, not dp.

This issue was detected using Coccinelle and the following semantic patch:

@@
expression x;
identifier fld;
@@

* x = devm_kzalloc(...);
  ... when != x == NULL
  x->fld

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170706215833.GA25411@embeddedgus