pvr: Remove PVR_WINSYS_BO_FLAG_ZERO_ON_ALLOC flag

pvr: Remove PVR_WINSYS_BO_FLAG_ZERO_ON_ALLOC flag

There has been a recent change to the new powervr KMD to always zero buffer
objects at allocation time to avoid information leaks. This change was made to
address upstream feedback [1]. The result is that the
PVR_WINSYS_BO_FLAG_ZERO_ON_ALLOC no longer makes a difference when using this
KMD.

As the powervr KMD is the one we actually care about, it makes sense to mirror
this change when using the downstream pvrsrvkm KMD in order to avoid differences
in behaviour between the two KMDs. As this makes the
PVR_WINSYS_BO_FLAG_ZERO_ON_ALLOC flag entirely redundant, remove it.

[1] https://lists.freedesktop.org/archives/dri-devel/2023-August/418042.html

Signed-off-by: Vlad Schiller <vlad-radu.schiller@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24930>