This reverts commit 446f2545.

I've left the masking in the pageflip code since that seems to be some
useful piece of preemptive robustness.

Iirc I've merged this patch under the assumption that the BIOS leaves
some random gunk in the lower bits and gets unhappy if we trample on
them. We have quite a few case like this, so this made sense.

Now I've just learned that there's actual hardware features bits in
the low 12 bits, and the kernel needs to preserve them to allow a
userspace blob to do its job. Given Dave Airlie's clear stance on
userspace blob drivers I've quickly chatted with him and he doesn't
seem too happy. So let's revert this.

If there are indeed bits that we must preserve in this range then we
can ressurrect this patch, but with proper documentation for those
bits supplied. And we probably also need to think a bit about
interactions with our driver.

Cc: Armin Reese <armin.c.reese@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Dave Airlie <airlied@linux.ie>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>