drm/i915: Add state readout for DP HDR Metadata Infoframe SDP

Added state readout for DP HDR Metadata Infoframe SDP.

v9: Rebased
v10: Rebased

Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514060732.3378396-9-gwan-gyeong.mun@intel.com