drm/i915/dp: Move max. vswing check to it's own function

Wrap the max. vswing check in a separate function.
This makes the clock recovery phase of DP link training cleaner

v3:
Fixed the paranthesis warning (Mika Kahola)
v2:
Fixed the Compiler warning (Mika Kahola)

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>