i965/fs: Use the GRF for FB writes on gen >= 7

   On gen 7, the MRF was removed and we gained the ability to do send
   instructions directly from the GRF.  This commit enables that
   functinoality for FB writes.

   v2: Make handling of components more sane.

i965/fs: Force a high register for the final FB write

   v2: Renamed the array for the range mappings and added a comment

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
6 files changed