drm/i915: Attempt to fix watermark setup on 85x (v2)

IS_MOBILE() catches 85x, so we'd always try to use the 9xx FIFO sizing;
since there's an explicit 85x version, this seems wrong.

v2: Handle 830m correctly too.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>