drm/prime: Allow internal imports without import_sg_table

Currently drm_gem_prime_import() checks if gem_prime_import_sg_table()
is implemented in DRM driver ops. However it is not necessary for
internal imports (i.e. dma_buf->ops == &drm_gem_prime_dmabuf_ops
and obj->dev == dev), which only increment reference count on respective
GEM objects.

This patch makes the helper check this condition only in case of
external imports fo rwhich importing sg table is indeed needed.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>