drm/i915: add proper CPU/PCH checks to crtc_mode_set functions

On ironlake_crtc_mode_set, WARN if not using IBX or CPT.

On haswell_crtc_mode_set, only run IBX/CPT code on IBX/CPT. I am still
not sure whether IBX/CPT will be possible with a Haswell CPU, so leave
the code there for now and put a WARN in case we spot it.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>