broadcom/vc4: Native fence fd support

broadcom/vc4: Native fence fd support

With the syncobj support in place, lets use it to implement the
EGL_ANDROID_native_fence_sync extension. This mostly follows previous
implementations in freedreno and etnaviv.

v2: Drop the flags (Eric)
    Handle in_fence_fd already in job_submit (Eric)
    Drop extra vc4_fence_context_init (Eric)
    Dup fds with CLOEXEC (Eric)
    Mention exact extension name (Eric)

Signed-off-by: Stefan Schake <stschake@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>