drm/ttm: Convert vm callbacks to helpers

The default TTM fault handler may not be completely sufficient
(vmwgfx needs to do some bookkeeping, control the write protectionand also
needs to restrict the number of prefaults).

Also make it possible replicate ttm_bo_vm_reserve() functionality for,
for example, mkwrite handlers.

So turn the TTM vm code into helpers: ttm_bo_vm_fault_reserved(),
ttm_bo_vm_open(), ttm_bo_vm_close() and ttm_bo_vm_reserve(). Also provide
a default TTM fault handler for other drivers to use.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Christian König <christian.koenig@amd.com>

include/drm/ttm/ttm_bo_api.h

@@ -727,4 +727,18 @@ static inline bool ttm_bo_uses_embedded_gem_object(struct ttm_buffer_object *bo)
 {
 	return bo->base.dev != NULL;
 }
+
+/* Default number of pre-faulted pages in the TTM fault handler */
+#define TTM_BO_VM_NUM_PREFAULT 16
+
+vm_fault_t ttm_bo_vm_reserve(struct ttm_buffer_object *bo,
+			     struct vm_fault *vmf);
+
+vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
+				    pgprot_t prot,
+				    pgoff_t num_prefault);
+
+void ttm_bo_vm_open(struct vm_area_struct *vma);
+
+void ttm_bo_vm_close(struct vm_area_struct *vma);
 #endif