drm/i915: Introduce intel_crtc_readout_derived_state()

Collect up a bunch of derived state "readout" into
a common helper, which we can call from both
intel_encoder_get_config() and intel_crtc_get_pipe_config().

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/20201112191718.16683-6-ville.syrjala@linux.intel.com