drm/i915: Move PTE_READ_ONLY to ->pte_encode vfunc

It's only used as a flag there, so unconfuse things a bit.
Also separate the bind_vma flag space from the pte_encode flag
space in the code.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>