i915/dp/fec: Add fec_enable to the crtc state.

For DP 1.4 and above, Display Stream compression can be
enabled only if Forward Error Correctin can be performed.

Add a crtc state for FEC. Currently, the state
is determined by platform, DP and DSC being
enabled. Moving forward we can use the state
to have error correction on other scenarios too
if needed.

v2:
- Control compression_enable with the fec_enable
parameter in crtc state and with intel_dp_supports_fec()
(Ville)

- intel_dp_can_fec()/intel_dp_supports_fec()(manasi)

v3: Check for FEC support along with setting crtc state.

v4: add checks to intel_dp_source_supports_dsc.(manasi)
- Move intel_dp_supports_fec() closer to
intel_dp_supports_dsc() (Anusha)

v5: Move fec check to intel_dp_supports_dsc(Ville)

v6: Remove warning. rebase.

v7: change crtc state to include DP sink and fec capability
of source.(Manasi)

v8: Set fec_enable in crtc in intel_dp_compute_config().

v9 (From Manasi):
* Combine the !edp and !fec_support check
* Derive dev_priv from intel_dp directly

v10 (From Manasi):
* Rebase

Suggested-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.comk>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181128202628.20238-14-manasi.d.navare@intel.com