gpu: drm: radeon: Adding new typedef vm_fault_t

Use new return type vm_fault_t for fault handler
in vm_operations_struct.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>