drm/i915: Update bits per component for display info

DisplayPort branch device may define max supported bits per
component. Update display info based on this value if bpc
is defined.

v2: cleanup to match the drm_dp_helper.c patches introduced
    earlier in this series
v3: Fill bpc for connector's display info in separate
    drm_dp_helper function (Daniel)
v4: remove updating bpc for display info as it may be overridden
    when parsing EDID. Instead, check bpc for DP branch device
    during compute_config
v5: Indentation fixes (Jim Bride)
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473419458-17080-11-git-send-email-mika.kahola@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -1524,6 +1524,20 @@ void intel_dp_compute_rate(struct intel_dp *intel_dp, int port_clock,
 	}
 }
 
+int intel_dp_compute_bpp(struct intel_dp *intel_dp,
+			 struct intel_crtc_state *pipe_config)
+{
+	int bpp, bpc;
+
+	bpp = pipe_config->pipe_bpp;
+	bpc = drm_dp_downstream_max_bpc(intel_dp->dpcd, intel_dp->downstream_ports);
+
+	if (bpc > 0)
+		bpp = min(bpp, 3*bpc);
+
+	return bpp;
+}
+
 bool
 intel_dp_compute_config(struct intel_encoder *encoder,
 			struct intel_crtc_state *pipe_config,
@@ -1590,7 +1604,7 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 
 	/* Walk through all bpp values. Luckily they're all nicely spaced with 2
 	 * bpc in between. */
-	bpp = pipe_config->pipe_bpp;
+	bpp = intel_dp_compute_bpp(intel_dp, pipe_config);
 	if (is_edp(intel_dp)) {
 
 		/* Get bpp from vbt only for panels that dont have bpp in edid */