i965: Disable depth writes when depth test is GL_EQUAL.

i965: Disable depth writes when depth test is GL_EQUAL.

There's no point in performing depth writes when the depth test
comparison function is set to GL_EQUAL - it would just write out
the same value that's already there (if it is written at all).  While
this is harmless from a functional perspective, it hurts performance.
Obviously, writing to memory is not free, but there's another more
subtle impact as well: it can prevent early depth optimizations.

Depth writes aren't supposed to happen for pixels that are killed
by fragment shader discard statements or the alpha test.  So, with
depth writes enabled and either of those, the pixel shader must be
invoked to determine whether or not to perform the write.  This is
fairly stupid in the EQUAL case - we're running a shader to decide
whether to replace the existing depth value with itself.

By disabling these pointless writes, we allow early depth even with
discards and alpha testing, allowing the hardware to skip the pixel
shader altogether if the depth test fails.

Improves performance of Unigine Valley:

- Skylake GT2:    +17.8%
- Broadwell GT3e: +11.5%
- Cherrytrail:    +19.4%

Huge thanks to Mark Janes for building frameretrace [1], the performance
analysis tool that helped us find this issue, and to Robert Bragg for
providing us performance metrics on Linux.  Mark also spent the time to
analyze Valley performance on Windows vs. Linux and discovered a
discrepancy in early depth test metrics.  Once he had isolated a draw
call and drawn attention to the problem, fixing it was pretty simple.

[1] https://github.com/janesma/apitrace/wiki/frameretrace-branch

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>