i965: Fix register write checks.

i965: Fix register write checks.

When mapping the buffer a second time, we need to use the new pointer,
not the one from the previous mapping.  Otherwise, we will most likely
crash.

Apparently, we've just been getting lucky and getting the same
bo->virtual pointer in both cases.  libdrm probably has a hand in that.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Cc: mesa-stable@lists.freedesktop.org