drm/i915/display: move struct intel_link_m_n to intel_display_types.h

struct intel_crtc_state in intel_display_types.h actually needs the
struct intel_link_m_n definition, while intel_display.h only needs the
forward declaration.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1ec10e4415cf84c51b7eb51092e81876da0bc902.1667383630.git.jani.nikula@intel.com