drm/amd: move variable to local scope

`edp_stream` is only used when backend is enabled on eDP, don't
declare the variable outside that scope.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>