drm/udl: properly check for error pointers

The drm_prime_pages_to_sg() function never returns NULL pointers, only
error pointers and valid pointers.

BUG=chromium:402599
TEST=emerge-link_freon chromeos-kernel-3_8
Signed-off-by: Haixia Shi <hshi@chromium.org>

Reviewed-on: https://chromium-review.googlesource.com/231704
Tested-by: Haixia Shi <hshi@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Queue: Haixia Shi <hshi@chromium.org>
(cherry picked from commit a7890d717e188ff8f6f404f2319778425314a5dd)

Change-Id: I3e47819c678b6a89ed6c135203da4f731970b64a
Reviewed-on: https://chromium-review.googlesource.com/232452
Reviewed-by: Haixia Shi <hshi@chromium.org>
Commit-Queue: Haixia Shi <hshi@chromium.org>
Tested-by: Haixia Shi <hshi@chromium.org>