i965/gen4-5: Cope with immutable-format texture revalidation

This is unfortunately sometimes necessary due to rebasing levels when
rendering into them.

16 piglits crash -> pass, when building mesa with debug enabled.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
1 file changed