i965: Use the upload BO for push constants on Gen9+.

i965: Use the upload BO for push constants on Gen9+.

Shaders can use quite a bit of uniform data.  Better to put it in the
upload buffers, like we do for client vertex data, rather than the
batch buffer state area, which is primarly used for indirect state.

This should free up batch space, allowing us to emit more commands in a
batch before flushing.  Because BRW_NEW_BATCH also causes a lot of state
to be re-emitted, it may also reduce CPU overhead a little bit.

We took this approach on Gen4-5, but switched to using the batch area
on Gen6+ because buffer 0 is relative to Dynamic State Base Address by
default, which is set to the start of the batch.

On Gen9+, we already use a relocation due to a workaround, so this is
trivial to change and has basically no downside.

Unfortunately we can't change compute shader push constants because
MEDIA_CURBE_LOAD always uses an offset from dynamic state base address.

Improves performance in GLBenchmark 2.7/TRex Offscreen by:
- Skylake GT4e: 0.52821% +/- 0.113402% (n = 190)
- Apollolake: 0.510225% +/- 0.273064% (n = 70)

Reviewed-by: Chris Forbes <chrisforbes@google.com>