drm/i915: OR in the COMMAND read domain for the batch buffer.

The batch buffer may be shared with another read buffer, so we should not
ignore any previously set domains, but just or in the command domain (and
check that the buffer is not writable).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>