drm/ttm: fix missing res assignment in ttm_range_man_alloc

That somehow got missing.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: cb1c8146 ("drm/ttm: flip the switch for driver allocated resources v2")
Link: https://patchwork.freedesktop.org/patch/msgid/20210607104040.22017-1-christian.koenig@amd.com