drm/i915: Make DP link training channel equalization DP 1.2 Spec compliant

Fix the number of tries in channel euqalization link training sequence
according to DP 1.2 Spec. It returns a boolean depending on channel
equalization pass or failure.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>