anv: leave the top 4Gb of the high heap VMA unused

anv: leave the top 4Gb of the high heap VMA unused

In 628c9ca9089789 I forgot to apply the same -4Gb of the high address
of the high heap VMA. This was previously computed in the
HIGH_HEAP_MAX_ADDRESS.

Many thanks to James for pointing this out.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reported-by: Xiong, James <james.xiong@intel.com>
Fixes: 628c9ca9089789 ("anv: store heap address bounds when initializing physical device")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>