drm/i915: Give proper names to MOCS entries

The purpose for each MOCS entry isn't well defined atm. Defining these
is important to remove any uncertainty about the use of these entries
for example in terms of performance and GPU/CPU coherency.

Suggested by Ville.

v4:
- Rename I915_MOCS_AUTO to I915_MOCS_PTE. (Chris)

CC: Rong R Yang <rong.r.yang@intel.com>
CC: Yakui Zhao <yakui.zhao@intel.com>
CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467383528-16142-1-git-send-email-imre.deak@intel.com