drm/i915/icl: implement the display init/uninit sequences

This code is similar enough to the CNL code that I considered just
adding ICL support to the CNL function, but I think it's still
different enough, and having a function specific to ICL allows us to
more easily adapt code in case the spec changes more later.

We're still missing the power wells and the mbus code, so leave those
pieces with a FIXME comment while they're not here yet.

v2: Don't use _PICK, don't WARN_ON(1), don't forget the chicken bits.
v3: Use _MMIO_PORT() (Ville).

Reviewed-by: James Ausmus <james.ausmus@intel.com> (v2)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205154046.11485-4-paulo.r.zanoni@intel.com