i965/fs: Remove brw_wm_compile::computes_depth field.

i965/fs: Remove brw_wm_compile::computes_depth field.

Everybody determines this by checking if fp's OutputsWritten field
contains the FRAG_RESULT_DEPTH bit.  Rather than having payload setup
check this and set the computes_depth flag, we can just do the check in
the only place that actually used it: emit_fb_writes().

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>