drm/i915/dsc: Add Per connector debugfs node for DSC support/enable

DSC can be supported per DP connector. This patch adds a per connector
debugfs node to expose DSC support capability by the kernel.
The same node can be used from userspace to force DSC enable.

force_dsc_en written through this debugfs node is used to force
DSC even for lower resolutions.

Credits to Ville Syrjala for suggesting the proper locks to be used
and to Lyude Paul for explaining how to use them in this context

v8:
* Add else if (ret) for drm_modeset_lock (Lyude)
v7:
* Get crtc, crtc_state from connector atomic state
and add proper locks and backoff (Ville, Chris Wilson, Lyude)
(Suggested-by: Ville Syrjala <ville.syrjala@linux.intel.com&gt;)>
* Use %zu for printing size_t variable (Lyude)
v6:
* Read fec_capable only for non edp (Manasi)
v5:
* Name it dsc sink support and also add
fec support in the same node (Ville)
v4:
* Add missed connector_status check (Manasi)
* Create i915_dsc_support node only for Gen >=10 (manasi)
* Access intel_dp->dsc_dpcd only if its not NULL (Manasi)
v3:
* Combine Force_dsc_en with this patch (Ville)
v2:
* Use kstrtobool_from_user to avoid explicit error checking (Lyude)
* Rebase on drm-tip (Manasi)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181206005407.4698-1-manasi.d.navare@intel.com