drm/xe/xe_pagefault: Disallow writes to read-only VMAs

In the Linux kernel, the following vulnerability has been resolved:

drm/xe/xe_pagefault: Disallow writes to read-only VMAs

The page fault handler should reject write/atomic access to read only

VMAs. Add code to handle this in xe_pagefault_service after the VMA

lookup.

v2:

- Apply max line length (Matthew)

(cherry picked from commit 714ee6754ac5fa3dc078856a196a6b124cd797a0)