zink: Fix MSVC RTC in zink_get_framebuffer_imageless()

zink: Fix MSVC RTC in zink_get_framebuffer_imageless()

The bit fields in zink_framebuffer_state cause a false positive with
MSVC's run-time checks enabled.  setting state.num_attachments in
zink_get_framebuffer_imageless().  Writing some bits of num_attachments
involves reading bits from layers and samples that haven't been
initialized.

Fixed by assigning to num_attachments earlier in the function.  Not
quite sure why that makes a difference but at a guess there's a
heuristic that considers assignment close to declaration as
initialization.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14566>