drm/mgag200: remove unneeded aper->count assignment after alloc_apertures()

alloc_apertures() already does the assignment for us, so assigning the
count member after the alloc_apertures() call is not needed.

Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>