drm/i915: Validate mode against max. link data rate for DP MST

Not validating the mode rate against max. link rate results in not pruning
invalid modes. For e.g, a HBR2 5.4 Gbps 2-lane configuration does not
support 4k@60Hz. But, we do not reject this mode.

So, make use of the helpers in intel_dp to validate mode data rate against
max. link data rate of a configuration.

v3: Renamed local variables again for consistency (Manasi)
v2: Renamed mode data rate local variable to be more explanatory.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479243546-17189-1-git-send-email-dhinakaran.pandiyan@intel.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/intel_drv.h

@@ -1457,6 +1457,8 @@ bool intel_dp_read_dpcd(struct intel_dp *intel_dp);
 bool __intel_dp_read_desc(struct intel_dp *intel_dp,
 			  struct intel_dp_desc *desc);
 bool intel_dp_read_desc(struct intel_dp *intel_dp);
+int intel_dp_link_required(int pixel_clock, int bpp);
+int intel_dp_max_data_rate(int max_link_clock, int max_lanes);
 
 /* intel_dp_aux_backlight.c */
 int intel_dp_aux_init_backlight_funcs(struct intel_connector *intel_connector);