i965/vec4: make the generator set correct NibCtrl for SIMD4 DF instructions

i965/vec4: make the generator set correct NibCtrl for SIMD4 DF instructions

From the HSW PRM, Command Reference, QtrCtrl:

   "NibCtrl is only allowed for SIMD4 instructions with a DF (Double Float)
    source or destination type."

v2: Assert that the type is DF (Samuel)
v3: Don't set the default group to 0 and then set it only for 4-wide
    instructions. Instead, assert that exec size and group are always
    a correct match and then always set the default group from the
    instruction. (Curro)

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