drm: replace weird conditional includes

drm: replace weird conditional includes

pte_wrprotect() is only used by drm_vm.c, so move the include there. Also
include it unconditionally, all architectures provide this header!

Furthermore, replace asm/current.h with sched.h, which includes
asm/current.h unconditionally. This way we get the same effect and avoid
direct asm/ includes. Furthermore, drop the weird __alpha__ protection.
It's safe to include sched.h everywhere (and the wait.h comment doesn't
apply, anyway).

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>