drm/amdkfd: Use kfd fd to mmap mmio

TTM doesn't support CPU mapping of sg type bo (under which
mmio bo is created). Switch mmaping of mmio page to kfd
device file.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Acked-by: Christian Konig <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>