drm/amdgpu: Implement ttm_bo_driver.access_memory callback v2

Allows gdb to access contents of user mode mapped VRAM BOs.

v2: return error for non-VRAM pools

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