i965/fs: fix MOV_INDIRECT exec_size for doubles

i965/fs: fix MOV_INDIRECT exec_size for doubles

In that case, the writes need two times the size of a 32-bit value.
We need to adjust the exec_size, so it is not breaking any hardware
rule.

v2:
  - Add an assert to verify type size is not less than 4 bytes (Jordan).

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>