drm/i915: Add state readout for DP VSC SDP

Added state readout for DP VSC SDP and enabled state validation
for DP VSC SDP.

v2: Minor style fix
v3: Replace a structure name to drm_dp_vsc_sdp from intel_dp_vsc_sdp
v4: Use struct drm_device logging macros
v10: Skip checking of VSC SDP when a crtc config has psr.

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/20200514060732.3378396-10-gwan-gyeong.mun@intel.com