Revert "drm/i915: Kill GTT mappings when moving from GTT domain"

Revert "drm/i915: Kill GTT mappings when moving from GTT domain"

This reverts commit 9a1d3f2f2bce4db232539fd10436e26bcec2cd1d.
Userland has always been required to set the object's domain to GTT
before using it through a GTT mapping, it's not something that the
kernel is supposed to enforce.  (The pagefault support is so that we
can handle multiple mappings without userland having to pin across
them, not so that userland can use GTT after GPU domains without
telling the kernel).

Fixes 19.2% +/- 0.8% (n=6) performance regression in cairo-gl
firefox-talos-gfx on my T420 latop.

Signed-off-by: Keith Packard <keithp@keithp.com>