drm/msm: return an error pointer in msm_gem_prime_get_sg_table()

drm/msm: return an error pointer in msm_gem_prime_get_sg_table()

The msm_gem_prime_get_sg_table() needs to return error pointers on
error.  This is called from drm_gem_map_dma_buf() and returning a
NULL will lead to a crash in that function.

Fixes: 78a3b53e9de2 ("drm/msm: fix msm_gem_prime_get_sg_table()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/485023/
Link: https://lore.kernel.org/r/YnOmtS5tfENywR9m@kili
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>