drm/i915: DVO pixel clock check

It is possible the we request to have a mode that has
higher pixel clock than our HW can support. This patch
checks if requested pixel clock is lower than the one
supported by the HW. The requested mode is discarded
if we cannot support the requested pixel clock.

This patch applies to DVO.

V2:
- removed computation for max pixel clock

V3:
- cleanup by removing unnecessary lines

V4:
- clock check against max dotclock moved inside 'if (fixed_mode)'

V5:
- dot clock check against fixed_mode clock when available

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dvo.c

@@ -201,6 +201,8 @@ intel_dvo_mode_valid(struct drm_connector *connector,
 		     struct drm_display_mode *mode)
 {
 	struct intel_dvo *intel_dvo = intel_attached_dvo(connector);
+	int max_dotclk = to_i915(connector->dev)->max_dotclk_freq;
+	int target_clock = mode->clock;
 
 	if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
 		return MODE_NO_DBLESCAN;
@@ -212,8 +214,13 @@ intel_dvo_mode_valid(struct drm_connector *connector,
 			return MODE_PANEL;
 		if (mode->vdisplay > intel_dvo->panel_fixed_mode->vdisplay)
 			return MODE_PANEL;
+
+		target_clock = intel_dvo->panel_fixed_mode->clock;
 	}
 
+	if (target_clock > max_dotclk)
+		return MODE_CLOCK_HIGH;
+
 	return intel_dvo->dev.dev_ops->mode_valid(&intel_dvo->dev, mode);
 }