i965/fs: Use brw_imm_uw().

i965/fs: Use brw_imm_uw().

W/UW immediates are 16-bits, but those 16-bits must be replicated
in the high 16-bits of the 32-bit field.

Remove the useless W/UW immediate saturating code, since we'll now be
using the appropriate immediate (and W/UW immediates in the IR can now
no longer be larger than 16-bits).

Reviewed-by: Emil Velikov <emil.velikov@collabora.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>