i965/fs: add a helper function to create double immediates

i965/fs: add a helper function to create double immediates

Gen7 hardware does not support double immediates so these need
to be moved in 32-bit chunks to a regular vgrf instead. Instead
of doing this every time we need to create a DF immediate,
create a helper function that does the right thing depending
on the hardware generation.

v2:
- Define setup_imm_df() as an independent function (Curro)
- Create a specific builder to get rid of some instruction field
  assignments (Curro).

v3:
- Get devinfo from builder (Kenneth)

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>