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

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: [email protected]
Cc: [email protected]
Cc: [email protected]
Reported-by: [email protected]
Reported-by: [email protected]
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93892
Fixes: 43d59eda1 ("drm/i915: Eliminate usage of plane_wm_parameters from ILK-style WM code (v2)")
Signed-off-by: Matt Roper <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]