i965: Determine size of double precision float register

i965: Determine size of double precision float register

This is used to determine how many registers an instruction reads and
writes as well as for offseting register region into a desired component.

v2 (Connor): rebase on master

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Signed-off-by: Tapani P\344lli <tapani.palli@intel.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>