drm/i915: Pretend cursor is always on for ILK-style WM calculations (v2)

Due to our lack of two-step watermark programming, our driver has
historically pretended that the cursor plane is always on for the
purpose of watermark calculations; this helps avoid serious flickering
when the cursor turns off/on (e.g., when the user moves the mouse
pointer to a different screen).  That workaround was accidentally
dropped as we started working toward atomic watermark updates.  Since we
still aren't quite there yet with two-stage updates, we need to
resurrect the workaround and treat the cursor as always active.

v2: Tweak cursor width calculations slightly to more closely match the
    logic we used before the atomic overhaul began.  (Ville)

Cc: simdev11@outlook.com
Cc: manfred.kitzbichler@gmail.com
Cc: drm-intel-fixes@lists.freedesktop.org
Reported-by:  <simdev11@outlook.com>
Reported-by:  <manfred.kitzbichler@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93892


Fixes: 43d59eda ("drm/i915: Eliminate usage of plane_wm_parameters from ILK-style WM code (v2)")
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454479611-6804-1-git-send-email-matthew.d.roper@intel.com