Revert "drm/i915: Force clean compilation with -Werror"

This reverts commit 118182e9.

It's causing too much trouble when compile-testing for non-i915 folks.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>