Revert r336326.

In testing on a Dell Latitude 7480, having ig4.ko loaded during a
suspend caused the system to hang. It turns out that ig4iic_intr() was
being called after the device entered D3, and entered an infinite loop
because a read of the I2C status register returned all ones, causing us
to attempt to read a byte from the data buffer until one of the status
bits clears. This occured because ig4iic_pci0 shares an interrupt with
the VGA device on this laptop, so ig4iic_intr() gets called even when
there is no work to do. This is exactly the problem fixed by r342170,
which resolves the hang for me and allows suspend/resume to work with
ig4.ko loaded. So, re-enable autoloading of ig4.ko in the hope that
r342170 resolves the problem universally.

Reviewed by: gonzo
MFC after: 1 month (pending an MFC of r342170)
Differential Revision: https://reviews.freebsd.org/D18587