drm/i915: cleanup the CHICKEN_MISC_2 (re)definitions

drm/i915: cleanup the CHICKEN_MISC_2 (re)definitions

* Don't define it twice.
* Define MSBs first, like the rest of i915_reg.h.
* Add CNL_ prefix to the bit that arrived in CNL.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170714175228.27019-1-paulo.r.zanoni@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>