drm/i915: extract set_m_n from ironlake_crtc_mode_set

The set_m_n code was spread all over the mode_set function.

Version 2:
Don't set the DP M/N registers on ironlake_set_m_n. Daniel Vetter has
plans to add some encoder-specific callbacks. Also, on this version we
don't change the order we're writing the registers, making the code
change safer.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>