drm/dp/fec: DRM helper for Forward Error Correction

DP 1.4 has Forward Error Correction Support(FEC).
Add helper function to check if the sink device
supports FEC.

v2: Separate the helper and the code that uses the helper into
two separate patches. (Manasi)

v3:
- Move the code to drm_dp_helper.c (Manasi)
- change the return type, code style changes (Gaurav)
- Use drm_dp_dpcd_readb instead of drm_dp_dpcd_read. (Jani)

v4:
- Avoid aux reads everytime, instead read cached
values of dpcd register (jani)
- Move helper to drm_dp_helper.h like other dsc
helpers.(Anusha)

v5: rebased. Change the helper parameter suitably.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Acked-by: Sean Paul <seanpaul@chromium.org> (For merging through
drm-intel)
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181102041455.15818-2-anusha.srivatsa@intel.com

include/drm/drm_dp_helper.h

@@ -1147,6 +1147,13 @@ drm_dp_dsc_sink_max_slice_width(const u8 dsc_dpcd[DP_DSC_RECEIVER_CAP_SIZE])
 		DP_DSC_SLICE_WIDTH_MULTIPLIER;
 }
 
+/* Forward Error Correction Support on DP 1.4 */
+static inline bool
+drm_dp_sink_supports_fec(const u8 fec_capable)
+{
+	return fec_capable & DP_FEC_CAPABLE;
+}
+
 /*
  * DisplayPort AUX channel
  */