drm.h: Fix DRM compilation with bare-metal toolchain.

An ifdef in drm.h expects to be compiled with full-fledged Linux
toolchain, but it's common to compile kernel with just bare-metal
toolchain which doesn't define __linux__. So, also add __KERNEL__
check.

[nm@ti.com: port forward to 3.9-rc6 and post to dri devel for feedback as RFC]
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>