drm/radeon: add radeon_bo_ref function

To be consistent with radeon_bo_unref, needed in the following patch.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/radeon_object.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_object.h
Simple merge