drm/amdgpu: Replace ttm_bo_reference with ttm_bo_get

The function ttm_bo_get acquires a reference on a TTM buffer object. The
function's name is more aligned to the Linux kernel convention of naming
ref-counting function _get and _put.

v2:
 * changed prefix to drm/amdgpu

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>