drm/i915: Simplify intel_get_crtc_ycbcr_config()

drm/i915: Simplify intel_get_crtc_ycbcr_config()

Make intel_get_crtc_ycbcr_config() simpler and rename it
to bdw_get_pipemisc_output_format() to better reflect what
it does.

Also toss in some comments to document that the 4:2:0 PIPECONF
bits are glk+ only. They are mbz on earlier platforms so reading
them unconditionally is safe however.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145053.25808-9-ville.syrjala@linux.intel.com
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>