drm: Add DP last received PSR SDP VSC register and bits

This is a register to help debug what is in the last SDP VSC
packet revived by sink.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328223046.16125-2-jose.souza@intel.com