How do I mount a TV bracket to a plasterboard/drywall ceiling?
If you are able to mount enough screws into the joists then that might be sufficient. 

However, you'll inevitably find that the joists aren't in the right place to match the screw holes in the bracket properly and using bolts will be more secure, as your tv is pretty heavy. 

Having access above is great though. Personally I would mount some study batons from above attached to the joists exactly where you want them, then bolt the bracket through the ceiling into the batons.

So...


find exactly where on the ceiling you want the bracket to go and mark on the ceiling where you want the bolt holes to go.
drill small holes through the ceiling at the marked points (you can then shove something like straws or bbq skewers into the holes and leave them there to make finding the holes easier from above).
go into the attic, find the holes and note their position.
get something like some 2x4 and make batons/a frame that will go between the joists, directly above the holes (making sure that the holes won't go at the edges of the batons).
Securely attach the batons to the joists, making sure that they are pressed right down against the plasterboard. (Exactly what the last two steps look like will depend on the positioning and alignment of your joists relative to the holes. Play it safe and go sturdier than you need, it'll be ugly but hidden in your attic).
go back down and drill the holes again going through your new wooden supports and wide enough for your bolts. 
bolt on the bracket. Get a friend with one of you above and below. Alternatively you can use something like coach bolts on to, but this will be trickier. 
mount tv. Sit back, relax and realise there's nothing on anyway.