drm/i915: Move PCH_NOP to -1

So we can later use PCH >= comparisons. The ultimate goal
is to make it easier for us to introduce a new platform
with south display engine on PCH just by reusing the previous
one.

Suggested-by: Lucas De Marchi <lucas.demarchi@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/20190308214300.25057-2-rodrigo.vivi@intel.com