drm/i915: Remove unused HAS_PCH_CNP_LP

Other than LPT, no other PCH needed to differentiate between
LP and HP. So let's remove this before we spread this mistake
to future platforms.

Cc: Anusha Srivatsa <anusha.srivatsa@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/20190221211716.9433-1-rodrigo.vivi@intel.com