drm/amdkfd: add helper function for kfd sysfs create

No functionality change. Modify kfd_sysfs_create_file to use kobject as
parameter, so it becomes common helper function to remove duplicate code
and will simplify new kfd sysfs file create in future.

Move pr_warn to helper function if sysfs file create failed. Set helper
function as void return because caller doesn't use the helper function
return value.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>