drm/amdgpu: add kernel config for gfx-userqueue

This patch:
- adds a kernel config option "CONFIG_DRM_AMDGPU_NAVI3X_USERQ"
- moves the usequeue initialization code for all IPs under
  this flag
- cover the core userqueue functions under this config
- adds stub function for userqueue ioctl.

so that the userqueue works only when the config is enabled.

V9:  Introduce this patch
V10: Call it CONFIG_DRM_AMDGPU_NAVI3X_USERQ instead of
     CONFIG_DRM_AMDGPU_USERQ_GFX (Christian)
V11: Add GFX in the config help description message.
V12: Add depends on BROKEN for this config, remove this when the rest of
the code is available.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Arvind Yadav <arvind.yadav@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>