i965/vec4: add a helper function to create double immediates

i965/vec4: 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 (Curro):
  - Use swizzle() and writemask() helpers and make tmp const.

v3 (Iago):
  - Adapt to changes in offset()

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>