broadcom/vc5: Move texture return channel setup into the compiler.

broadcom/vc5: Move texture return channel setup into the compiler.

The compiler decides how many LDTMUs we're going to emit, and that must
match the P1 flags.  This brings the return channel counting to a single
place (so all that's passed into the compiler is "how many return channels
you may request from this texture's format), and was a necessary step for
shadow samplers once we stop using OVRTMUOUT=0.