intel: fix length mask for Gen5/Gen6 3DSTATE_CLEAR_PARAMS

On Gen6, bit 15 is now `Depth Clear Value Valid`. This was being treated
as part of the length, and failing the rest of the batchbuffer decode.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>