iris: Hack up a SKL/Gen9LP PS push constant fifo depth workaround

iris: Hack up a SKL/Gen9LP PS push constant fifo depth workaround

This is a port of Nanley's 904c2a617d86944fbdc2c955f327aacd0b3df318
from i965 to iris.

One concern is that iris uses larger batches, and also emits far fewer
commands, so we may come closer to the 500 limit within a batch, and
could need to supplement this with actual counting.  Manhattan 3.0 had
239 3DSTATE_CONSTANT_PS packets in a batch,  Unigine Valley had 155.
So it seems like we're still in the realm of safety.