drm/amdgpu: Modify userq signal/wait struct field names

Modify kernel UAPI userq signal/wait struct field names and
description corresponding to the libdrm UAPI review comments.

libdrm MR: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/392



Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>