broadcom/compiler: Set up passthrough Z when doing FS discards.

broadcom/compiler: Set up passthrough Z when doing FS discards.

In order to keep early-Z from writing early in a discard shader, you need
to set the "modifies Z" bit in the shader state (which the new
prog_data.discards will indicate).  Then, in the shader we do a TLB write
to make Z passthrough happen (the QPU result is ignored, so we use a NULL
source).