drm/ttm: Implement vm_operations_struct.access v2

Allows gdb to access contents of user mode mapped BOs. System memory
is handled by TTM using kmap. Other memory pools require a new driver
callback in ttm_bo_driver.

v2:
* kmap only one page at a time
* swap in BO if needed
* make driver callback more generic to handle private memory pools
* document callback return value
* WARN_ON -> WARN_ON_ONCE

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>