i965: Move PSCDEPTH calculations from draw time to compile time.

i965: Move PSCDEPTH calculations from draw time to compile time.

The "Pixel Shader Computed Depth Mode" value is entirely based on the
shader program, so we can easily do it at compile time.  This avoids the
if+switch on every 3DSTATE_WM (Gen7)/3DSTATE_PS_EXTRA (Gen8+) upload,
and shares a bit more code.

This also simplifies the PMA stall code, making it match the formula
more closely, and drops a BRW_NEW_FRAGMENT_PROGRAM dependency.  (Note
that the previous comment was wrong - the code and the documentation
have != PSCDEPTH_OFF, not ==.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>