rootdir: init.rc: add drmrpc to surfaceflinger supplemental groups

This is necessary for some HWC hals to be able to communicate with
secure side to grant protected access to hardware owned by the
hwc. This is necessary on some architectures to grant access to
secure buffers to overlay/csc hardware

Change-Id: I4e4becba5b4a80310ce8932edea5d0d046fa9b00
Signed-off-by: Dima Zavin <dima@android.com>