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

include/drm/drm_dp_helper.h

@@ -477,6 +477,7 @@
 # define DP_PSR_FRAME_CAPTURE		    (1 << 3)
 # define DP_PSR_SELECTIVE_UPDATE	    (1 << 4)
 # define DP_PSR_IRQ_HPD_WITH_CRC_ERRORS     (1 << 5)
+# define DP_PSR_ENABLE_PSR2		    (1 << 6) /* eDP 1.4a */
 
 #define DP_ADAPTER_CTRL			    0x1a0
 # define DP_ADAPTER_CTRL_FORCE_LOAD_SENSE   (1 << 0)