drm/i915: Restore the preliminary HW check.

We still maintain code internally that cares about preliminary support.
Leaving the check here doesn't hurt anyone, and should keep things more
in line.

This time around, stick the info in the intel_info structure, and also
change the error from DRM_ERROR->DRM_INFO.

This is a partial revert of:
commit 590e4df8
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Wed May 8 10:45:15 2013 -0700

    drm/i915: VLV support is no longer preliminary

Daniel, I'll provide the fix ups for internal too if/when you merge
this (if you want).

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>