video: tegra: host: better error handling in alloc_gathers()

We should return -ENOMEM in alloc_gathers() when get a NULL pointer
from nvmap_alloc() or nvmap_mmap()

Bug 1178135

Change-Id: I29321710343983a6e733d95b10a1f7eb586246c0
Signed-off-by: Haley Teng <hteng@nvidia.com>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>

Conflicts:
	drivers/video/tegra/host/nvhost_job.c