drm/i915: i830M has watermarks like i855

So shuffle the checks around a bit. Also give all the structs and
functions proper prefixes: i830_ for the dual-pipe mobile platforms
and i845_ for the two single-pipe desktop platforms.

Note that the max fifo value isn't actually correct for the i830M, but
since we don't frob the fifo split we don't actually need it. This is
different for some gen3 devices where we need the full fifo for self
refresh mode.

Cc: Thomas Richter <richter@rus.uni-stuttgart.de>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>