drm: Add DP PSR2 sink enable bit

To comply with eDP1.4a this bit should be set when enabling PSR2.

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/20180328223046.16125-1-jose.souza@intel.com