mm, drm/udl: fixup vma flags on mmap

There should be VM_MIXEDMAP, not VM_PFNMAP, because udl_gem_fault() inserts
pages via vm_insert_page(). Other drm/gem drivers already do this.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Dave Airlie <airlied@redhat.com>