drm: Add DP PSR2 sink enable bit

drm: Add DP PSR2 sink enable bit

BugLink: http://bugs.launchpad.net/bugs/1815234
[ Upstream commit 4f212e40468650e220c1770876c7f25b8e0c1ff5 ]

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
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>