drm/ttm/tests: Use an init function from the helpers lib

drm/ttm/tests: Use an init function from the helpers lib

Add a new helper function that also initializes the device. Use it in
ttm_tt test suite and delete the local definition.

Signed-off-by: Karolina Stolarek <karolina.stolarek@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Somalapuram, Amaranath <asomalap@amd.com>
Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/235f110c1bdd2da7fac62ec03d177a5160dba1d5.1718192625.git.karolina.stolarek@intel.com