WA 0884:bxt:all,cnl:*:A - "When FBC is enabled with eDP PSR,
the CPU host modify writes may not get updated on the Display
as expected.
WA: Write 0x00000000 to CUR_SURFLIVE_A with every CPU
host modify write to trigger PSR exit."

We can also find on spec other cases where they describe
bogus writes to cursor registers to force PSR exit with
HW tracking. And it was confirmed by HW engineers that
this Wa can be safely applied for any frontbuffer activity.

So let's use this more and more here instead of forcibly
disable and re-enable PSR everytime that we have a simple
reliable flush case.

Other commits improve the fbcon/fbdev use a lot, but this
approach is the only when where we can get a fully reliable
console with no slowness or missed frames and PSR still
enabled and active.

v2: - Rebase on drm-tip
    - (DK) Add a comment to explain that WA
    tells about writing 0 to CUR_SURFLIVE_A but we write to
    CUR_SURFLIVE(pipe).
v3: Wa doesn't work on PSR2.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180309005218.26772-1-rodrigo.vivi@intel.com