There is no rule that says that damage is all or nothing.
The spell says, "...On a failed save, the target takes 10d6+40 force damage..." Lets say your total is 80 points of damage.
The spell then says, "...If
this damage reduces the target to 0 hit points, it is disintegrated..."
'This damage' is 80 points. If you haven't applied all 80, then you haven't applied 'this damage'. There is no rule that even
allows you to do
less than the damage you rolled. Just like in blackjack you have no permission to add less than the full value of the drawn card.
You can only check for dust
if you have applied 'this damage'. You don't have permission to check for dust until you have applied 'this damage'.
This defines the 'order of operations' for this spell. Apply 'this damage',
then check to see if 'this damage' has reduced you to 0 hp. For our druid, his beast form has run out of hp and now he's into his own, but since his own hp have not been reduced to 0 hp, he is not dust.
At any time, in beast or druid form, he only has one hp total. But when the beast form has taken enough damage to lose all its hp, then there is no beast form with 0 hp; there is only a druid with whatever he has left. The 0 hp of the beast form is not his hp; the beast form's hp ceased to be the druid's hp as soon as they went below 1 hp. The druid has not been reduced to 0 hp, he has been reduced to...27, or whatever. When you check to see if 'this damage' has reduced the druid to 0 hp, the answer is no, 'this damage' has reduced him to 27 hp.
There is no rule allowing you to do less (or more) damage than the spell says it does, just because you want to. You must have an actual game mechanic that allows you to do that. You can't cast
fireball, roll 30 damage, then realise that you killed some innocent kids and sat, "Actually, I'll only do 3 damage instead!" You can't roll 80 damage for your
disintegrate spell and say, "Actually, I'll only do 53 damage instead!"
When you apply 'this damage' to the beast form, it
instantaneously eliminates the beast form, leaving a damaged (but still conscious) druid who has been reduced to 27 hp.