drm/i915: Fix ilk+ watermarks when disabling pipes

We're no longer programming any watermarks when we're disabling
a pipe. That means ilk_wm_merge() & co. will keep considering
the any pipe that is getting disabled as still enabled. Thus we
either get no LP1+ watermakrs (ilk-ivb), or we get suboptimal
ones (hsw-bdw).

This seems to have been broken by commit b6b178a7 ("drm/i915:
Calculate ironlake intermediate watermarks correctly, v2."). Before
that we apparently had some difference between the intermediate
and optimal watermarks and so we would program the optiomal ones.
Now intermediate and optimal are identical for disabled pipes
and so we don't program either.

Fix this by programming the intermediate watermarks even for
disabled pipes. We were already doing that for skl+. We'll
leave out gmch platforms for now since those do the merging
in a different manner and should work as is. We'll want to
unify this eventually, but play it safe for now and just put
in a FIXME.

Cc: stable@vger.kernel.org
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: b6b178a7

 ("drm/i915: Calculate ironlake intermediate watermarks correctly, v2.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181025130536.29024-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #irc