drm/i915: Pre-validate the NEED_GTTS flag for execbuffer

We have an implementation requirement that precludes the user from
requesting a ggtt entry when the device is operating in ppgtt mode. Move
the current check from inside the execbuffer object collation to the
prevalidation phase.

v2: Roll both invalid flags checks into one

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>