i965: Use the upload BO for push constants on Gen7.5-Gen8.

i965: Use the upload BO for push constants on Gen7.5-Gen8.

We can easily use the upload BO for push constants on Gen7.5/Gen8 too,
at the cost of a relocation when emitting 3DSTATE_CONSTANT_XS.  We can
simply switch to using constant buffer pointer 2 instead of pointer 0,
like we do on Gen9+.

Ivybridge and Baytrail can't do this trick because they require the
constant buffers to be enabled in order, starting with 0.  We'd have
to set the INSTPM bit to make the constant buffer pointer not relative
to dynamic state base address, which would need kernel command parser
support.

Improves performance in GLBenchmark 2.7/TRex Offscreen by:
- Broadwell GT2: 0.305608% +/- 0.19877% (n = 68)
- Braswell: No difference proven (n = 742)
- Haswell GT3e: 0.180755% +/- 0.0237505% (n = 30)

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