drm/i915: add a context parameter to {en, dis}able zero address mapping

Export a new context parameter that can be set/queried through the
context_{get,set}param ioctls.  This parameter is passed as a context
flag and decides whether or not a GPU address mapping is allowed to
be made at address zero.  The default is to allow such mappings.

Signed-off-by: David Weinehall <david.weinehall@intel.com>
Acked-by: "Zou, Nanhai" <nanhai.zou@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>