drm_fourcc: Fix DRM_FORMAT_MOD_LINEAR #define

We need to define DRM_FORMAT_MOD_VENDOR_NONE for the fourcc_mod_code()
macro to work correctly.

Signed-off-by: Kristian H. Kristensen <hoegsberg@google.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1481657272-25975-1-git-send-email-hoegsberg@google.com