drm/i915: Start using comparative INTEL_PCH_TYPE

In order to make it easier to bring up new platforms
without having to take care about all corner cases
that was previously taken care for previous platforms
we already use comparative INTEL_GEN statements.

Let's start doing the same with PCH.

The only caveats are:
 - less-than comparisons need to be avoided or done with
   attention and check > PCH_NONE as well.
 - It is not necessarily a chronological order, but a matter
   of south display compatibility/inheritance.

v2: Rebased on top of Jani's clean-up which removed the
    need for less-than comparison

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190308214300.25057-3-rodrigo.vivi@intel.com