i965: Set MaxIfDepth to UINT_MAX on Gen6+ and 16 on prior generations.

i965: Set MaxIfDepth to UINT_MAX on Gen6+ and 16 on prior generations.

Commit 488fe51cf823ccd137c667f1e92dd86f8323b723 converted the EmitNoIfs
flag to MaxIfDepth, an unsigned integer saying "flatten if-statements
nested beyond this depth."

Unfortunately, i965 left this initialized to 0, which made ir_to_mesa
attempt to flatten all if-statements.  We didn't notice right away
because we usually throw away ir_to_mesa's code in favor of the native
VS and FS backends...but this still creates a lot of unnecessary work.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>