i965/fs: Use 'c' as the mem_ctx in fs_visitor.

i965/fs: Use 'c' as the mem_ctx in fs_visitor.

Previously, the memory context situation was a bit of a mess:

fs_visitor allocated its own memory context, and freed it in the
destructor.  However, some data produced by fs_visitor (such as the list
of instructions) needs to live beyond when fs_visitor is "done", so the
caller can pass it to fs_generator.

Everything worked out because brw_wm_fs_emit's fs_visitor variables
happen to not go out of scope until the end of the function.  But that
meant that moving the declaration of, say, the SIMD16 fs_visitor
instance, could cause everything to explode.

Using a memory context that exists for the duration of the compile is
clearer, and should be equivalent.

Ultimately, we don't want to use 'c', but this matches the behavior of
fs_generator and gen8_fs_generator, so it'll be simple to change later.

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