drm: amdgpu: Use the correct size when allocating memory

When '*sgt' is allocated, we must allocated 'sizeof(**sgt)' bytes instead
of 'sizeof(*sg)'.

The sizeof(*sg) is bigger than sizeof(**sgt) so this wastes memory but
it won't lead to corruption.

Fixes: f44ffd67 ("drm/amdgpu: add support for exporting VRAM using DMA-buf v3")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>