We cannot let IPS enabled with no plane on the pipe:

BSpec: "IPS cannot be enabled until after at least one plane has
been enabled for at least one vertical blank." and "IPS must be
disabled while there is still at least one plane enabled on the
same pipe as IPS." This restriction apply to HSW and BDW.

However a shortcut path on update primary plane function
to make primary plane invisible by setting DSPCTRL to 0
was leting IPS enabled while there was no
other plane enabled on the pipe causing flickerings that we were
believing that it was caused by that other restriction where
ips cannot be used when pixel rate is greater than 95% of cdclok.

v2: Don't mess with Atomic path as pointed out by Ville.

v3: Rebase after a long time and atomic path changes.
    Accept Ville suggestion of not check !fb

v4: Re-factore on dinq

Reference: https://bugs.freedesktop.org/show_bug.cgi?id=85583
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Kenneth Graunke <kenneth@whitecape.org>
[danvet: Make it compile]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>