drm/rockchip: fix error check when getting irq

platform_get_irq() can return negative error values and we already test for
these. Therefore the variable holding this value should be signed to not
loose possible error values.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-By: Daniel Kurtz <djkurtz@chromium.org>