drm/amd/display: Change get_pipe_idx function scope

Turn previously global function into a static function as it is not used
outside the file.

Reviewed-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>