We need this for doing USB offload scenarios using glamor
and modesetting driver.
unfortunately only gbm in mesa 10.6 has support for the
linear API.
v1.1: fix bad define
v2: update the configure.ac test as per amdgpu. (Michel)
set linear bos to external to avoid cache. (Eric)
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>