drm/amdgpu: extend xnack limit page fault timeout

Extending this timeout will prevent IH from storm interrupts coming
from SDMA while a page fault is active. Currently, on Aldebaran,
handling that many interrupts can take a lot of CPU time
(up to 4 seconds).
This eventually causes timeouts in other tasks.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>