i965: Use <0,2,1> region for scalar DF sources on IVB/BYT.

i965: Use <0,2,1> region for scalar DF sources on IVB/BYT.

On HSW+, scalar DF sources can be accessed using the normal <0,1,0>
region, but on IVB and BYT DF regions must be programmed in terms of
floats. A <0,2,1> region accomplishes this.

v2:
- Apply region <0,2,1> in brw_reg_from_fs_reg() (Curro).

v3:
- Added comment explaining the reason (Curro).

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>