drm/i915: use TU_SIZE macro at intel_dp_set_m_n

Much simpler and looks more like the M/N code inside intel_display.c.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>