drm/i915: refactor stepping info retrieval

Have only one if ladder for platforms and only one range check for
size. Makes it easier to handle new platforms. Remove the use of
negative return values in char, which might underflow to be positive for
some negative error codes.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445344713-1407-3-git-send-email-jani.nikula@intel.com