drm/xe/oa: Use vma_pages() helper function in xe_oa_mmap()

Use the vma_pages() helper function and remove the following
Coccinelle/coccicheck warning reported by vma_pages.cocci:

  WARNING: Consider using vma_pages helper on vma

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240819095751.539645-2-thorsten.blum@toblux.com