drm/amdgpu: change job->ctx field name

job->ctx actually is a fence_context of the entity
it belongs to, naming it as ctx is too vague, and
we'll need add amdgpu_ctx into the job structure
later.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>