winsys/radeon: Use CPU page size instead of hardcoding 4096 bytes v3

winsys/radeon: Use CPU page size instead of hardcoding 4096 bytes v3

Fixes GPUVM conflicts with non-4K page size.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92738

v2: Replace sanitization of VM base address alignment with comment why
    that's not necessary.
v3: Use unsigned instead of long as the type for the size_align member.
    (Marek)

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Christian König <christian.koenig@amd.com> (v1)
Reviewed-by: Marek Olšák <marek.olsak@amd.com>