drm/i915: refuse to load on gen6+ without kms

Spurred by an irc discussion, let's start to clear up which parts of
our kms + ums/gem + ums/dri1 + vbios/dri1 kernel driver pieces
userspace in the wild actually uses.

The idea is that we introduce checks at entry-points (module load
time, ioctls, ...) first and then reap any obviously dead code in a
second step.

As a first step refuse to load without kms on chips where userspace
never supported ums. Now upstream hasn't supported ums on ilk, ever.
But RHEL had the great idea to backport the kms support to their ums
driver.

Cc: Dave Airlie <airlied@gmail.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>