drm/i915: Always read out M2_N2 in intel_cpu_transcoder_get_m_n, v2.

has_drrs is a flag we can't read out. We set it when seamless DRRS is
enabled in pipe_config, so intel_dump_pipe_config() and
intel_pipe_config_compare() will continue to do the right thing when
has_drrs is set on the real state.

This removes one more dereference of crtc->config.
While at it, fixup the comment and also read out M2_N2 for CHV, since
we program it in the set_m_n function.

Changes since v1:
- Only read out M2/N2 on platforms that support DRRS.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181015094023.6211-1-maarten.lankhorst@linux.intel.com