drm/xe: s/XE_PTE_READ_ONLY/XE_PTE_FLAG_READ_ONLY

This define is for internal PTE flags rather than fields in the hardware
PTEs, rename as such. This will help in an upcoming patch to avoid
further confusion.

Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>