drm/i915: s/intel_mode_from_pipe_config/intel_mode_from_crtc_timings/

Generalize intel_mode_from_pipe_config() to work on any two
arbitrary modes. Also relocate the code for the future, and
make it static since it's not needed elsewhere.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201112191718.16683-5-ville.syrjala@linux.intel.com