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

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)

Fixes: fb544b844508 ("drm/xe: Implement xe_pagefault_queue_work")
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Suggested-by: Matthew Brost <matthew.brost@intel.com>
Cc: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260324152935.72444-7-jonathan.cavitt@intel.com