drm/i915: add HAS_VEBOX

The flag will be useful to help share code between IVB, and HSW as the
programming is similar in many places with this as one of the major
differences.

Signed-off-by: Xiang, Haihao <haihao.xiang@intel.com>
[Commit message + small fix by]
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>