kdb, kms: Save and restore the LUT on atomic KMS enter/exit

When changing VTs non-atomically the kernel works in conjunction with
the Xserver in user space and receives the LUT information from the
Xserver via a system call.  When changing modes atomically for kdb,
this information must be saved and restored without disturbing user
space as if nothing ever happened.

There is a short cut used by this patch where gamma_store is used as
the save space.  If this turns out to be a problem in the future a
pre-allocated chunk of memory will be required for each crtc to save
and restore the LUT information.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
CC: Jesse Barnes <jbarnes@virtuousgeek.org>
CC: David Airlie <airlied@linux.ie>
CC: dri-devel@lists.freedesktop.org
Signed-off-by: Dave Airlie <airlied@redhat.com>