drm/amdkfd: remove an unused variable

Just for cleanup.

Reviewed-by: Prike Liang <Prike.Liang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>