drm/i915: Added write-enable pte bit supportt

This adds support for a write-enable bit in the entry of GTT.
This is handled via a read-only flag in the GEM buffer object which
is then used to see how to set the bit when writing the GTT entries.
Currently by default the Batch buffer & Ring buffers are marked as read only.

v2: Moved the pte override code for read-only bit to 'byt_pte_encode'. (Chris)
    Fixed the issue of leaving 'gt_old_ro' as unused. (Chris)

v3: Removed the 'gt_old_ro' field, now setting RO bit only for Ring Buffers(Daniel).

v4: Added a new 'flags' parameter to all the pte(gen6) encode & insert_entries functions,
    in lieu of overloading the cache_level enum (Daniel).

v5: Removed the superfluous VLV check & changed the definition location of PTE_READ_ONLY flag (Imre)

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>