drm: xlnx: dp: Reset DisplayPort IP

Assert DisplayPort reset signal before deasserting,
it is to clear out any registers programmed before booting kernel.

Signed-off-by: Rohit Visavalia <rohit.visavalia@amd.com>
Reviewed-by: Vishal Sagar <vishal.sagar@amd.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240216124043.1226713-1-rohit.visavalia@amd.com