On HSW/BDW with VT-d active the first tile row scanned out
after the first async flip of the frame often ends up corrupted.

Whether the corruption happens or not depends on the scanline
on which the async flip happens, but the behaviour seems very
consistent. Ie. the same set of scanlines (which are most scanlines)
always show the corruption. And another set of scanlines (far less
of them) never shows the corruption.

I discovered that disabling the fetch-stride stretching
feature cures the corruption. This is some kind of TLB related
prefetch thing AFAIK. We already disable it on SNB primary
planes due to a documented workaround. The hardware folks
indicated that disabling this should be fine, so let's go
with that.

And while we're here, let's document the relevant bits on all
pre-skl platforms.

Fixes: 2a636e24 ("drm/i915: Implement async flip for ivb/hsw")
Fixes: cda195f1 ("drm/i915: Implement async flips for bdw")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210220103303.3448-1-ville.syrjala@linux.intel.comReviewed-by: Karthik B S <karthik.b.s@intel.com>