vc4: Refactor qir_get_op_nsrc(enum qop) to qir_get_nsrc(struct qinst *).

vc4: Refactor qir_get_op_nsrc(enum qop) to qir_get_nsrc(struct qinst *).

Every caller was dereffing the qinst, and this will let us make the number
of sources vary depending on the destination of the qinst so that we can
have general ALU ops that store to tex_[strb] and get an implicit uniform.