drm/i915: Set DP min_bpp to 8*3 for non-RGB output formats

6bpc is only legal for RGB and RAW pixel encodings. For the rest
the minimum is 8bpc. Set our lower limit accordingly.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326142556.21176-6-ville.syrjala@linux.intel.com
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>

drivers/gpu/drm/i915/intel_dp.h

@@ -34,6 +34,7 @@ void intel_dp_adjust_compliance_config(struct intel_dp *intel_dp,
 				       struct link_config_limits *limits);
 bool intel_dp_limited_color_range(const struct intel_crtc_state *crtc_state,
 				  const struct drm_connector_state *conn_state);
+int intel_dp_min_bpp(const struct intel_crtc_state *crtc_state);
 bool intel_dp_port_enabled(struct drm_i915_private *dev_priv,
 			   i915_reg_t dp_reg, enum port port,
 			   enum pipe *pipe);