drm/i915: Use HAS_PCH_SPLIT to determine correct devices

drm/i915: Use HAS_PCH_SPLIT to determine correct devices

Do some further clean up based on the initial review of
drm/i915: Separate cherryview from valleyview.

In this case check for '(GEN7 || GEN8) && HAS_PCH_SPLIT' since we're
reading FUSE_STRAP and SFUSE_STRAP which live in the PCH.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Wayne Boyer <wayne.boyer@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449514270-15171-3-git-send-email-wayne.boyer@intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>