mxge: Move busdma information for RX done ring out of the hot path

These information is rarely accessed
mxge: Remove cnt from RX done ring struct; which serves no purpose
mxge: Nuke the flag field in mxge_tx_buffer_state

Instead of using flag to mark the end of the TX packet chain, we now
save mbuf in the last tx_buffer_state and use the mbuf pointer to mark
the end of the TX packet.
mxge: mxge_tx_buffer_state and mxge_rx_buffer_state are same now; merge them
mxge: Cache align TX and RX struct
drm: Remove unused files
drm: Reduce differences with Linux a bit

* drm_internal.h is no longer present in Linux
mxge: Mark TX queue no longer over-active earlier
mxge: Add macro for RX small buf length