drm/gt215/gr: fix initialisation on gddr5 boards

The binary driver modifies the default context to have this value, rather
than 0x3d0040, *after* it's filled the buffer with the usual golden data.

We don't really have anything in place to locate the correct offset to do
these type of modifications outside of the generation function, so this
will have to do.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>