drmPrime*: initialize output args to 0

Fix Valgrind errors because those memory was uninitialized.

https://bugs.freedesktop.org/show_bug.cgi?id=90194


Signed-off-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>

v2: Explicitly zero the whole struct using memclear.
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>