gallium/util: do not pass undefined sample-count

We forgot to initialize the sample_count member here, leading to it being undefined. This causes problems on MSVC when compiling in debug-mode, where we get a run-time error for using an undefined variable.

To avoid similar problems in the future if more fields are added, let's initialize the whole struct to zero to start with. This also allows us to remove a no-longer-needed zero-initialization.