i965/fs: Replace c->key with a direct reference in fs_visitor.

i965/fs: Replace c->key with a direct reference in fs_visitor.

'c' is going away.  This is also shorter.

Marking the key pointer as const will also deter people from changing
it in fs_visitor, as it's absolutely not OK to modify it there.

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