i965/fs: Handle zero-size allocations in fs_builder::vgrf().

i965/fs: Handle zero-size allocations in fs_builder::vgrf().

This will be handy to avoid some ugly ternary operators in the next
patch, like:
 fs_reg reg = (size == 0 ? null_reg_ud() : vgrf(..., size));

Because a zero-size register allocation is guaranteed not to ever be
read or written we can just return the null register.  Another
possibility would be to actually allocate a zero-size VGRF what would
involve defining a zero-size register class in the register allocator
and a considerable amount of churn.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>