drm/i915: CRT 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 CRT.

V2:
- removed computation for max pixel clock

V3:
- cleanup by removing unnecessary lines

V4:
- max_pixclk variable renamed as max_dotclk

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>
Link: http://patchwork.freedesktop.org/patch/msgid/1454419003-6001-6-git-send-email-mika.kahola@intel.com

drivers/gpu/drm/i915/intel_crt.c

@@ -216,6 +216,7 @@ intel_crt_mode_valid(struct drm_connector *connector,
 		     struct drm_display_mode *mode)
 {
 	struct drm_device *dev = connector->dev;
+	int max_dotclk = to_i915(dev)->max_dotclk_freq;
 
 	int max_clock = 0;
 	if (mode->flags & DRM_MODE_FLAG_DBLSCAN)
@@ -231,6 +232,9 @@ intel_crt_mode_valid(struct drm_connector *connector,
 	if (mode->clock > max_clock)
 		return MODE_CLOCK_HIGH;
 
+	if (mode->clock > max_dotclk)
+		return MODE_CLOCK_HIGH;
+
 	/* The FDI receiver on LPT only supports 8bpc and only has 2 lanes. */
 	if (HAS_PCH_LPT(dev) &&
 	    (ironlake_get_lanes_required(mode->clock, 270000, 24) > 2))