i965: Change the hiz-override env var to a driconf option.

The force-enable option is dropped, now that the hardware we were
concerned about has HiZ on by default.  Now, instead of doing
INTEL_HIZ=0 to test disabling hiz, you can set hiz=false.

v2: Disable separate stencil on gen6 when HIZ is turned off.
    (previously, this had to be done manually in addition).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
3 files changed