AMDGPU: Fix enqueue block lowering for opaque pointers

This was looking for a specific constant cast of the function, when
the type doesn't matter. Doesn't bother trying to handle typed
pointers, it will just assert.

Things probably don't work completely correctly if the block kernel
address is captured somewhere else, but that wouldn't work before
either. The uses should really be loads out of the handle, and the
handle initializer should contain the kernel address.