radeonsi: add missing initialization for userptr buffers

radeonsi: add missing initialization for userptr buffers

Fix the accounting for memory usage of userptr buffers, which has been wrong
forever (or at least for a long time).

Also initialize flags. Without this initialization, the sparse buffer flag
might end up being set, which leads to staging buffers being used unnecessarily
(and incorrectly) in transfers to or from userptr buffers.

This works around VM faults that occur with the radeon kernel module when
running piglit ./bin/amd_pinned_memory decrement-offset map-buffer -auto

Fixes: e077c5fe6579 ("gallium/radeon: transfers and invalidation for sparse buffers")
Reported-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>