i965/fs: Replace fs_reg(imm) constructors with brw_imm_*().

i965/fs: Replace fs_reg(imm) constructors with brw_imm_*().

Cuts 10k of .text, of which only 776 bytes are the fs_reg constructor
implementations themselves.

   text     data      bss      dec      hex  filename
5204535   214112    27784  5446431   531b1f  i965_dri.so before
5193977   214112    27784  5435873   52f1e1  i965_dri.so after

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