drm/i915: Move the NULL sg handling out from rotate_pages()

rotate_pages() checks to see if it got called with a NULL sg, and then
goes to extract it from sg->sgl. It always gets called with a NULL sg
for the first plane, so moving the initial 'sg=st->sgl' assignment out
into intel_rotate_fb_obj_pages() seems less special-casey.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455569699-27905-9-git-send-email-ville.syrjala@linux.intel.com