drm/amdgpu: use explicit limit for VRAM_CONTIGUOUS

Split VRAM won't have a valid offset, so just set an explicit limit
when the flag is given to trigger reallocation if necessary.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Tested-by: Mike Lothian <mike@fireburn.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>