drm/amdgpu: add entity error check in amdgpu_ctx_get_entity

[Why]
UMD is not aware of entity error, and will keep submitting jobs
into the error entity.

[How]
Add entity error check when getting entity from ctx.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: ZhenGuo Yin <zhenguo.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>