drm/ttm: user reservation object wrappers v2

Consistently use the reservation object wrappers instead of accessing
the ww_mutex directly.

Additional to that use the reservation object wrappers directly instead of
calling __ttm_bo_reserve with fixed parameters.

v2: fix typo

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>