drm/amdgpu: remove PL_PREEMPT accounting

This is provided by TTM now.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214093439.2989-7-christian.koenig@amd.com