drm/i915: Reject invalid-pad for context-destroy and -create ioctls

Unknown parameters, especially structure padding, are expected to invoke
rejection with -EINVAL.

v2: similar issue exists for context-create

Testcase: igt/gem_ctx_create/invalid-pad
Testcase: igt/gem_ctx_bad_destroy/invalid-pad
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89602
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93999
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1454690759-31201-1-git-send-email-chris@chris-wilson.co.uk