drm: kill context_ctor callback

It's not used by any driver. The destructor callback is unfortunately
used by the via driver in a rather convoluted piece of code used
to reimplement something resembling broken futexes. I didn't dare
to touch this code. But at least kill the needless NULL assignemt
in the sis driver.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>