drm: define __linux__ for gcc arm-eabi-4.6

this fixes following error,

>   CC [M]  drivers/gpu/drm/drm_auth.o
> In file included from include/drm/drmP.h:76:0,
>                  from drivers/gpu/drm/drm_auth.c:36:
> include/drm/drm.h:47:24: fatal error: sys/ioccom.h: No such file or directory
> compilation terminated.
> make[3]: *** [drivers/gpu/drm/drm_auth.o] Error 1

arm-eabi-4.6 is the default compiler for Rockchip 3.0 kernel.