intel: PAGE_SIZE used in allocators shouldn't be defined on FreeBSD

intel: PAGE_SIZE used in allocators shouldn't be defined on FreeBSD

iris, i965, and anv define the PAGE_SIZE in anv_allocator and bufmgr
files. As on FreeBSD the page size is defined in machine/param.h that is
indirectly included by those files, we'd rather define it only when the
system is not FreeBSD to avoid compile errors.

v2: Changed the path in the comment to make clear that machine/params.h
is a FreeBSD system file.

Signed-off-by: Eleni Maria Stea <elene.mst@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11203>