drm/tegra: nvdec: Fix dma_alloc_coherent error check

drm/tegra: nvdec: Fix dma_alloc_coherent error check

[ Upstream commit 44306a684cd1699b8562a54945ddc43e2abc9eab ]

Check for NULL return value with dma_alloc_coherent, in line with
Robin's fix for vic.c in 'drm/tegra: vic: Fix DMA API misuse'.

Fixes: 46f226c93d35 ("drm/tegra: Add NVDEC driver")
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20250702-nvdec-dma-error-check-v1-1-c388b402c53a@nvidia.com
Signed-off-by: Sasha Levin <sashal@kernel.org>