drm/ttm: fix pipelined gutting v2

We need to make sure to allocate the sys_mem resource before the point
of no return.

v2: add missing return value checking, also handle idle case

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210608081931.11339-1-christian.koenig@amd.com