drm/i915: set FORCE_ARB_IDLE_PLANES workaround

Commit 1544d9d5 added a workaround
inside haswell_init_clock_gating and mentioned it is "a workaround for
early silicon revisions and should be removed later". This workaround
is documented in bit 31 of PRI_CTL. I asked Arthur and he mentioned
that setting FORCE_ARB_IDLE_PLANES replaces that workaround for the
newer machines. So use the new one.

Also notice that there's still another workaround for PRI_CTL that
involves WM_DBG, but it's not the one we're reverting. And notice that
we were previously setting WM_DBG_DISALLOW_MULTIPIPE_LP which disables
the LP watermarks when more than one pipe is used, and we really don't
want this because we need the LP watermarks if we want to reach deeper
PC states.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Add a comment for the w/a name Ville dug out of Bspec.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>